Method for imparting improved fatigue strength to wire made of shape memory alloys, and medical devices made from such wire

ABSTRACT

Wire products, such as round and flat wire, strands, cables, and tubing, are made from a shape memory material in which inherent defects within the material are isolated from the bulk material phase of the material within one or more stabilized material phases, such that the wire product demonstrates improved fatigue resistance. In one application, a method of mechanical conditioning in accordance with the present disclosure isolates inherent defects in nickel-titanium or NiTi materials in fields of a secondary material phase that are resistant to crack initiation and/or propagation, such as a martensite phase, while the remainder of the surrounding defect-free material remains in a primary or parent material phase, such as an austenite phase, whereby the overall superelastic nature of the material is preserved.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. Provisional Patent Application Ser.No. 61/110,084, entitled NANOGRAIN DAMAGE RESISTANT WIRE, filed on Oct.31, 2008, U.S. Provisional Patent Application Ser. No. 61/179,558,entitled NANOGRAIN DAMAGE RESISTANT WIRE, filed on May 19, 2009, andU.S. Provisional Patent Application Ser. No. 61/228,677, entitledNANOGRAIN DAMAGE RESISTANT WIRE, filed on Jul. 27, 2009, the entiredisclosures of which are expressly incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to fatigue damage resistant wire and, inparticular, relates to a method of manufacturing wire made of a shapememory alloy, which demonstrates improved fatigue strength properties,as well as medical devices made with such wire.

2. Description of the Related Art

Shape memory materials are materials that “remember” their originalshape, and which, after being deformed, return to that shape eitherspontaneously or by applying heat to raise their temperature above aprocessing and material related threshold known as the transformationtemperature. Heating to recover shape is commonly referred to in the artas “shape memory”, whereas spontaneous recovery is commonly referred toas pseudoelasticity. Pseudoelasticity, sometimes called superelasticity,is a reversible response to an applied stress, caused by a phasetransformation between the austenite or parent phase and the martensiteor daughter phase of a crystal. It is exhibited in shape memory alloys.Pseudoelasticity and shape memory both arise from the reversible motionof domain boundaries during the phase transformation, rather than justbond stretching or the introduction of defects in the crystal lattice. Apseudoelastic material may return to its previous shape after theremoval of even relatively high applied strains by heating. For example,even if the secondary or daughter domain boundaries do become pinned,for example due to dislocations associated with plasticity, they may bereverted to the primary or parent phase by stresses generated throughheating. Examples of shape memory materials include iron-chrome-nickel,iron-manganese, iron-palladium, iron-platinum,iron-nickel-cobalt-titanium, iron-nickel-cobalt-tantalum-aluminum-boron,copper-zinc-aluminum, copper-zinc-aluminum-nickel,copper-aluminum-nickel, and nickel-titanium alloys. Shape memorymaterials can also be alloyed with other materials including zinc,copper, gold, and iron.

Shape memory materials are presently used in a variety of applications.For example, a variety of military, medical, safety and roboticapplications for shape memory materials are known. Medical grade shapememory materials are used for orthodontic wires, guide wires to guidecatheters through blood vessels, surgical anchoring devices and stentapplications, for example. One shape memory material in wide use,particularly in medical device applications, is a nickel-titanium shapememory material known as “Nitinol”.

Many medical grade shape memory wire products are made of biocompatibleimplant grade materials including “NiTi” materials. As used herein,“nickel-titanium material”, “nickel-titanium shape memory material” and“NiTi” refer to the family of nickel-titanium shape memory materialsincluding Nitinol (an approximately equiatomic nickel-titanium, binaryshape memory material) as well as alloys including nickel and titaniumas primary constituents but which also include one or more additionalelements as secondary constituents, such as Nitinol tertiary orquaternary alloys (Nitinol with additive metals such as chromium,tantalum, palladium, platinum, iron, cobalt, tungsten, iridium andgold).

Significant research has been dedicated to understanding how NiTibehaves in the body from the viewpoint of biological host response, butmuch less has been published that quantitatively correlates structurewith mechanical properties.

More particularly, the fatigue properties of NiTi material have been thesubject of recent research. The fatigue crack propagation behavior ofNitinol was studied in detail by McKelvey and Ritchie, as published inFatigue-Crack Growth Behavior in the Superelastic and Shape-Memory AlloyNitinol, Metallurgical and Materials Transactions, 32A, 2001, pgs.731-743. McKelvey et al. observed that the crack growth propagation rateand ΔK_(th), which denotes the stress-intensity fatigue threshold in agiven fatigue-crack growth scenario, were different for equivalentcomposition at martensite-stable and austenite-stable temperatures wherethe crack growth rate was generally lower at martensite-stabletemperatures. They also observed that, under plane strain conditions,the heavily slipped material near the crack tip at superelastic regimetemperatures remained austenitic, presumably inhibited from undergoingvolume contractile, stress-induced phase transformation by the triaxialstress state, while plane stress conditions generally resulted instress-induced martensite near the crack tip.

Wire products made of shape memory materials are manufactured by forminga relatively thick piece of hot-worked rod stock from a melt process.The rod stock is then further processed into wires by drawing the rodstock down to a thin diameter wire. During a drawing process, oftenreferred to as a “cold working” process, a wire is pulled through alubricated die to reduce its diameter. The deformation associated withwire drawing increases the stress in the material, and the stresseventually must be relieved by various methods of heat treatment orannealing at elevated temperatures to restore ductility, thus enablingthe material to be further cold worked to a smaller diameter.Conventional wire annealing typically results in grain growth with aconcomitant random crystal orientation, and the various material orfiber “textures” that are generated during cold wire drawing are mostlyeliminated during conventional annealing and recrystallization. Theseiterative processes of cold working and annealing may be repeatedseveral times before a wire of a desired diameter is produced andprocessing is completed.

Wire materials manufactured by the above processes typically containmicrostructural defects, such as pores, inclusions, interstitials, anddislocations. An inclusion comprises a phase which possesses distinctproperties from the primary material matrix and is divided from thematrix by a phase boundary. Inclusions may result from oxide or othermetallic or non-metallic precipitate formation during primary melting orother high temperature treatment and may include carbides, nitrides,silicides, oxides or other types of particles. Inclusions may also arisefrom contamination of the primary melt materials or from the mold whichcontains the molten ingot. In the case of an interstitial, an atomoccupies a site in the crystal structure at which there usually is notan atom. The atom may be a part of its host material, such as a basemetal or alloying metal, or it may be an impurity. A dislocation is alinear defect around which some of the atoms of the crystal lattice aremisaligned and appear as either edge dislocations or screw dislocations.Edge dislocations are caused by the termination of a plane of atoms inthe middle of a crystal, while a screw dislocation comprises an internalstructure in which a helical path is traced around the linear defect ordislocation line by the atomic planes of atoms in the crystal lattice.Mixed dislocations, combining aspects of screw and edge dislocations,may also occur.

Internal or external defects, such as inclusions, pores, or defectsinduced during wire processing may weaken the host material at the siteof the defect, potentially resulting in failure of a material at thesite of that defect. This weakening may be particularly acute where thedefect is relatively large and/or of significantly disparate stiffnesscompared with adjacent dimensions of the material (such as for fine orsmall diameter wire). Failure of shape memory wires is more likely tooccur at the site of the defect. Since inherent defects cannot becompletely eliminated from the wire material, management of inherentdefects and mitigation of their negative impact on wire properties isdesirable.

One previously proposed solution to the problem of inherent defects hasbeen to treat selected regions of a wire that are expected to besubjected to high strain by converting the bulk material in such regionsto a different phase than the remainder of the bulk material of thewire. For example, under predetermined operating conditions, such as apredetermined operation temperature, the high strain wire regions arestabilized in a martensite phase while the lesser strain regions remainin an austenite phase. This method is therefore directed to treatingpredetermined regions of a wire to convert the bulk material in theregions to a more stable phase regardless of the presence, number, andlocation of any defects in the bulk material.

However, it may not always be possible or practical to predict whatregions of a continuous wire will be subjected to high strains whenportions of the wire are later incorporated into a medical device. Itmay also be desirable to leave defect-free portions of wire unaffectedby mitigation efforts and, therefore, available to meet other designconsiderations. For example, a disadvantage of the above process is thatfor wire made of shape memory material, the regions that are stabilizedin the martensite phase will lose the superelastic characteristic.

Although wires made in accordance with foregoing processes maydemonstrate excellent fatigue strength, further improvements in fatiguestrength are desired, particularly with reference to fatigue damage thatpropagates from defects.

What is needed is a method of manufacturing a wire that demonstratesimproved fatigue strength, and medical devices that include such wire.

SUMMARY

The present disclosure relates to wire products, and medical devicesincluding wire products, such as round and flat wire, strands, cables,coils, and tubing, made from a shape memory material or alloy. Defectswithin the material are isolated from a primary, or parent, materialphase within one or more areas of stabilized secondary, or daughter,material phases that are resistant to failure, such that the wireproduct demonstrates improved fatigue strength. In one application, amethod of mechanical conditioning in accordance with the presentdisclosure isolates defects in nickel-titanium or NiTi shape memorymaterials in localized areas or fields of a secondary material phasethat are resistant to crack initiation and/or propagation, such as amartensite phase, while the remainder of the surrounding defect-freematerial remains in a primary material phase, such as an austenitephase, whereby the overall superelastic and/or nature of the material ispreserved.

Wire products manufactured in accordance with the present disclosuremaintain good mechanical properties in addition to improved fatigueperformance Increases in the strain fatigue limit for both high cycleand low cycle fatigue are observed, while shape memory or superelasticcharacteristics are preserved.

As discussed below and shown in the Working Examples, the amount ofsecondary phase material formed about the defects during the mechanicalconditioning process is sufficient to either completely isolate thedefects or at least partially isolate high stress concentrator areasabout the defects in order to the improve fatigue strength of thematerial and yet, when the bulk of the material reverts back to theprimary phase after the mechanical conditioning, the overall amount ofremaining secondary phase material that is formed about the defects isnot sufficient compromise the shape memory or superelasticcharacteristic of the material as a whole. In this respect, the amountof mechanical conditioning may be specifically tailored to achieve adesired balance between fatigue strength and material elasticity.

In one form thereof, the present invention provides a medical deviceincluding a wire made of a nickel-titanium shape memory material, thewire having a fatigue endurance exceeding 0.95% strain amplitude atgreater than 10⁶ cycles.

In other embodiments, the medical device may include a wire having afatigue endurance exceeding 1.1% strain amplitude at greater than 10⁶cycles, or a fatigue endurance exceeding 1.1% strain amplitude atgreater than 10⁹ cycles. In a further embodiment, the medical device mayinclude a wire having a residual strain of less than 0.25% after beingsubjected to engineering strain of at least 9.5%.

In another form thereof, the present invention provides a medical deviceincluding a wire product made of a shape memory material, the shapememory material having a plurality of defects, the wire productsubstantially comprised of the shape memory material in a primary phaseand including portions of the shape memory material comprising asecondary phase at localized regions disposed proximate respectivedefects, with at least some of the secondary phase portions separated bythe primary phase.

The shape memory material may be a nickel-titanium shape memorymaterial, in which the primary phase is an austenite phase, and thesecondary phase portions comprise a martensite phase. The secondaryphase portions may together comprise less than 15% of the shape memorymaterial, by volume.

In a further embodiment, the shape memory material may be anickel-titanium shape memory material, with the wire product having afatigue endurance exceeding 0.95% strain amplitude at greater than 10⁶cycles, a fatigue endurance exceeding 1.1% strain amplitude at greaterthan 10⁶ cycles, or a fatigue endurance exceeding 1.1% strain amplitudeat greater than 10⁹ cycles. The wire may also have a residual strain ofless than 0.25% after being subjected to engineering strain of at least9.5%. The wire product may be selected from the group consisting of wirehaving a circular cross-section, wire having a non-circularcross-section, cable, coil, and tubing.

In a further form thereof, the present invention provides a method,including the steps of: providing a wire product made of a shape-set,shape memory material; mechanically conditioning the wire product by:applying an engineering stress between 700 MPa and 1600 MPa; andreleasing the applied engineering stress; and incorporating the wireproduct into a medical device. The mechanical conditioning step mayoccur either prior to or after the incorporation step.

In another embodiment, the mechanical conditioning step includes:applying an engineering stress between 900 MPa and 1450 MPa; andreleasing the applied engineering stress. In a further embodiment, themechanical conditioning step includes: applying an engineering stressbetween 1100 MPa and 1350 MPa; and releasing the applied engineeringstress. The method may further include the repeating the mechanicallyconditioning step at least once.

In one embodiment, the shape memory material may be a nickel-titaniumshape memory material, and the mechanical conditioning step may beconducted below a martensite deformation temperature (M_(d)) of thenickel-titanium shape memory material. The mechanical conditioning stepmay further include: applying the first force to the wire product in anenvironment having a temperature T, wherein

T=A _(f)±50° C.,

wherein A_(f) is the austenite transformation finish temperature of thenickel-titanium shape memory material. The wire product may be selectedfrom the group consisting of wire having a circular cross-section, wirehaving a non-circular cross-section, cable, coil, and tubing.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescriptions of embodiments of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a schematic view of a portion of wire having an equiaxed grainstructure;

FIG. 2 is a schematic view of the portion of wire of FIG. 1 having anelongated grain structure after cold work conditioning;

FIG. 3 is a schematic view of the portion of wire of FIG. 2 having anequiaxed grain structure with smaller grains than the equiaxed grainstructure of the wire in FIG. 1 after a shape set annealing process;

FIG. 4 is a schematic view illustrating an exemplary drawing processusing a lubricated die;

FIG. 5 is a depiction of the processing step of conditioning a wireusing a mechanical conditioning method in accordance with the presentdisclosure;

FIG. 6 is a depiction of the processing step of releasing the tension inthe wire of FIG. 6;

FIG. 7( a) is a view of a portion of a wire having internal and externaldefects;

FIG. 7( b) is an fragmentary view of a defect in the wire of FIG. 7( a);

FIG. 8 is a stress-strain curve for a mechanical conditioning process inaccordance with the present disclosure;

FIG. 9 is a view of a portion of a wire having internal and externaldefects substantially surrounded by dislocation-stabilized secondaryphase;

FIG. 10( a) is a fragmentary view of a defect in the wire of FIG. 9substantially surrounded by dislocation-stabilized secondary phase;

FIG. 10( b) is a view of a portion of a wire having internal andexternal defects substantially surrounded by dislocation-stabilizedsecondary phase;

FIG. 10( c) is a view of a portion of a wire having internal andexternal defects substantially surrounded by dislocation-stabilizedsecondary phase;

FIG. 11 is a graphical strain-life representation of rotary beam fatiguedata generated in accordance with Example 1 under the following testconditions: R=−1, T=298 K, f=60 s⁻¹; environment: quiescent air, N=5 ateach strain level;

FIG. 12( a) is a secondary electron (SE) image of a 100 nm deep cue markfor optical determination of defect zone;

FIG. 12( b) is an image of a transversely oriented, 10×3×0.5 μm (T×R×L)FIB-milled sharp defect;

FIG. 12( c) is an image that provides an overall view of a sharp defectzone;

FIG. 12( d) is an optical photograph of 150 μm diameter NiTi wires withcue marks evident near centerline;

FIG. 12( e) is a transverse SEM micrograph of a failed fatigue fracturespecimen showing the FIB-milled sharp defect (FSD) depth correspondingto FIG. 12( b);

FIG. 13( a) shows graphical representations of cyclic tensile data forsamples including overload conditioning cycle, non-conditioned, andconditioned samples, with varying test temperatures;

FIG. 13( b) is an enlarged insets showing a loading region of the graphshown in FIG. 13( a);

FIG. 13( c) is an enlarged insets showing an unloading region of thegraph shown in FIG. 13( a);

FIG. 14 is a graphical representation of rotary bend fatigue data forconditioned (C) and non-conditioned (NC) samples under the followingtest conditions: T=300 K, rate=60 s⁻¹, R=−1, with a maximum stresserror=3% and a maximum cycle count error=0.5%;

FIG. 15 is a graphical representation of single test level (1%alternating engineering strain) data for FIB-sharp defect (FSD) andFSD-conditioned (FSD-C) samples, with the extension bars in the insetrepresenting the data spread for n=3 samples;

FIG. 16 is a bright field TEM (BF-TEM) image of an FSD crack root aftermechanical conditioning, with the insets showing selected area electrondiffraction patterns (SADP) for regions within (left) and outside of(right) the structurally distinct zone demarcated by a dashed line andextending approximately 0.5 nm from the crack tip;

FIG. 17 is a graphical representation of crack growth rate data inferredfrom high resolution scanning electron microscopy of ductile striationspacing observations and estimated stress intensity at probable crackfront location based on a semi-elliptical crack in an infinite rod;

FIG. 18( a) is a graph showing cycles to failure for five sets of wiresamples, where a sample from each set of wires has been mechanicallyconditioned with a given level of engineering stress, and where thewires were tested at a 1.25% strain level;

FIG. 18( b) is a graph showing cycles to failure for the five sets ofwire samples shown in FIG. 18( a), where a sample from each set of wireshas been mechanically conditioned with a given level of engineeringstress, and where the wires were tested at a 1.1% strain level;

FIG. 18( c) is a graph showing cycles to failure for the five sets ofwire samples shown in FIG. 18( a), where a sample from each set of wireshas been mechanically conditioned with a given level of engineeringstress, and where the wires were tested at a 0.95% strain level;

FIG. 18( d) is a graph showing cycles to failure for the five sets ofwire samples shown in FIG. 18( a), where a sample from each set of wireshas been mechanically conditioned with a given level of engineeringstress, and where the wires were tested at a 0.80% strain level;

FIG. 19( a) is a stress-strain curve for five wire samples, where eachwire sample was loaded using the conditioning regime indicated by thelegend at the right of the figure and described in Table 2;

FIG. 19( b) is a stress-strain curve for five wire samples, where eachwire sample was loaded using the conditioning regime indicated by thelegend at the right of the figure and described in Table 2;

FIG. 19( c) is a stress-strain curve for five wire samples, where eachwire sample was loaded using the conditioning regime indicated by thelegend at the right of the figure and described in Table 2;

FIG. 19( d) is a stress-strain curve for five wire samples, where eachwire sample was loaded using the conditioning regime indicated by thelegend at the right of the figure and described in Table 2;

FIG. 19( e) is a stress-strain curve for five wire samples, where eachwire sample was loaded using the conditioning regime indicated by thelegend at the right of the figure and described in Table 2;

FIG. 20 is a graph showing the percentage of isothermallynon-recoverable strain in various wire materials as a function of amechanical conditioning parameter;

FIG. 21( a) is a section view of Drawn Filled Tubing (DFT®) wiremanufactured in accordance with an embodiment of the present disclosure(DFT® is a registered trademark of Fort Wayne Metals Research ProductsCorporation of Fort Wayne, Ind.);

FIG. 21( b) is a cross sectional view taken along line 18B-18B of FIG.18( a);

FIG. 22( a) is an elevation view of a braided tissue scaffold or stentincluding a wire made in accordance with the present process; and

FIG. 22( b) is an elevation view of a knitted tissue scaffold or stentincluding a wire made in accordance with the present process.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate preferred embodiments of the invention and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION

The present disclosure relates to wire products, and medical devicesincluding wire products, such as round and flat wire, strands, cables,coils, and tubing, made from a shape memory material or alloy. Defectswithin the material are isolated from a primary, or parent, materialphase within one or more stabilized secondary, or daughter, materialphases that are resistant to failure, such that the wire productdemonstrates improved fatigue strength. In one application, a method ofmechanical conditioning in accordance with the present disclosureisolates defects in nickel-titanium or NiTi shape memory materials inlocalized areas or fields of a secondary material phase that areresistant to crack initiation and/or propagation, such as a martensitephase, while the remainder of the surrounding defect-free materialremains in a primary material phase, such as an austenite phase, wherebythe overall superelastic nature of the material is preserved.

As used herein, a “defect” refers to material defects such as crack-likedefects, inclusions, dislocations, and other non-uniformities, as wellas any other internal or external defects or stress risers present in amaterial, as well as melt intrinsic and extrinsic defects such asinclusions, porosity, voids and oxide precipitate formation aftermelting.

Exemplary manufacturing processes by which wires may be made inaccordance with the present disclosure are set forth in Section I below,and general descriptions of the resulting physical characteristics ofwires made in accordance with the present process are set forth inSection II below. Working Examples are set forth in Section III below.Applications using wires made in accordance with the present disclosureare set forth in Section IV below.

Several suitable shape memory materials may be used for forming wireproducts according to the present disclosure. As used herein, “shapememory material” encompasses medical grade shape memory materialsincluding nickel-titanium or NiTi (defined above), as well as medicalgrade shape memory alloys including beta titanium alloys (such as Beta Cthat comprise primarily the beta phase at room temperature), and anyother medical grade shape memory alloys exhibiting similar superelasticand/or shape memory characteristics such as tantalum-titanium,titanium-niobium, and iron-nickel-cobalt alloys. Additionally, as usedherein, “shape memory material” also encompasses non-medical grade shapememory alloys such as iron-chrome-nickel, iron-manganese,iron-palladium, iron-platinum, iron-nickel-cobalt-titanium,iron-nickel-cobalt-tantalum-aluminum-boron, copper-zinc-aluminum,copper-zinc-aluminum-nickel, copper-aluminum-nickel, and nickel-titaniumalloys.

Moreover, it is contemplated that various shape-memory materials havingeither a one-way memory effect or a two-way memory effect, and otherrelated materials, may be subjected to the present mechanicalconditioning process to achieve enhanced physical characteristicsidentified in the discussion below and in the corresponding WorkingExamples.

As discussed in detail in Section IV below, fatigue damage resistantshape memory wire made in accordance with the present disclosure may beused in medical devices such as, for example, implantable cardiacpacing, shocking and/or sensing leads, implantable neurologicalstimulating and/or sensing leads, wire-based stents, blood filterdevices, or any other medical device application in which high fatiguestrength and/or a shape memory or superelastic characteristic isdesired. Wire products produced in accordance with the presentdisclosure may also be used in non-medical device applications in whichhigh fatigue strength and/or a shape memory or superelasticcharacteristic is desired.

As used herein, “wire” or “wire product” encompasses continuous wire andwire products, such as wire having a round cross section and wire havinga non-round cross section, including flat wire, as well as otherwire-based products such as strands, cables, coil, and tubing.

I. DESCRIPTION OF THE PRESENT MANUFACTURING PROCESS

1. Wire Preparation

Prior to the mechanical conditioning process of the present disclosure,discussed below, wire made of a shape memory material is subjected tocold work prior to undergoing a shape set annealing process. The shapesetting step imparts the primary shape memory and/or superelasticcharacteristics of the material prior to mechanical conditioning.

Initial preparation of a wire may involve first forming a piece of rodstock, for example, based on conventional melt processing techniques,followed by one or more iterations of conventional cold working andannealing. Referring to FIG. 1, a schematic or exaggerated view of aportion of wire 10 manufactured in accordance with conventional coldworking and annealing techniques is shown. Wire 10 has been subjected toone or more, perhaps several or a very large number of, iterations ofconventional cold working and annealing, as described above, to form anequiaxed crystal structure within the material of wire 10.Representative equiaxed crystals are depicted in wire 10 at 12. As usedherein, “equiaxed” refers to a crystal structure in which the individualcrystals 12 have axes that are approximately the same length, such thatthe crystals 12 collectively have a large number of slip planes, leadingto high strength and ductility. However, it is not necessary that thegrain structure be equiaxed. The grain structure may, for example,contain deformed grains that have been recovered to the B2 cubicaustenite phase through the high temperature shape setting processdescribed below.

Referring now to FIG. 2, prior to the shape-set anneal, wire 10 mayoptionally subjected to further cold work in the form of a cold workconditioning step if a nanograin microstructure is desired. As usedherein, “cold work conditioning” means imparting a relatively largeamount of cold work to a material, such as by wire drawing, swaging, orotherwise forming.

Referring to FIG. 4, the cold work conditioning step is performed bydrawing wire 10 through a lubricated die 18 (FIG. 4) having a an outputdiameter D₂, which is less than diameter D₁ of the undrawn wire 10 shownin FIG. 2. In one exemplary embodiment, the cold work conditioning stepby which the diameter of wire 10 is reduced from D₁ to D₂ is performedin a single draw and, in another embodiment, the cold work conditioningstep by which the diameter of wire 10 is reduced from D₁ to D₂ isperformed in multiple draws which are performed sequentially without anyannealing step therebetween.

Further discussion of exemplary cold work conditioning processes arepresented in U.S. patent application Ser. No. 12/563,062, entitledFATIGUE DAMAGE RESISTANT WIRE AND METHOD OF PRODUCTION THEREOF, filedSep. 18, 2009, assigned to the present assignee, the disclosure of whichis hereby expressly incorporated by reference herein in its entirety.The foregoing reference also discloses methods of limited annealingfollowing the cold work conditioning to create a nanograinmicrostructure, which may optionally be applied to wires prior tosubjecting same to the mechanical conditioning process in accordancewith the present disclosure, discussed below.

Regardless of the amount of cold work imparted to the wire and/orwhether cold work conditioning is used, once drawn to the desired size,and as shown in FIG. 5, wire 10 undergoes a shape setting annealingprocess in which it is continuously annealed under constant tensionsufficient to hold the wire in a substantially linear configurationduring the shape set annealing process. Shape set annealing typicallyoccurs at a temperature between 300° C. (673 K) and 600° C. (1073 K),where the temperature is sufficiently high to restore a majority of thewire material to the primary or austenite phase. The shape settingannealing process may result in the formation of a new crystallographicstructure, which may comprise nano-scale equiaxed crystals 16 in wire10, as shown in FIG. 3. However, as noted above, an equiaxed ornanograin crystal microstructure is not required for the mechanicalconditioning process described below.

2. Mechanical Conditioning

In accordance with the present disclosure, wire made of a shape memorymaterial may be subjected to a mechanical conditioning process toimprove its resistance to fatigue damage. In the present embodiment,mechanical conditioning is performed by application of a force to thewire in an environment having a temperature range within approximately50° C. of the austenite transformation finish temperature (A_(f)), i.e.,T=A_(f)±50° C.

Referring now to FIGS. 5 and 6, in the mechanical conditioning process,shape memory wire, shown in FIG. 5, in the heat set annealed conditionis secured at a first end. Controlled engineering strain is applied inthe direction of arrow F to a finished engineering strain, ε_(c), of0.08 to 0.14 units, wherein:

${ɛ_{c} = \frac{\Delta \; L}{L_{o}}},$

with L_(o) being the initial length, and ΔL being the length increaseimparted by strain.

The range for ε_(c) may be from as little as 0.06, 0.08, or 0.10 unitsto as much as 0.13, 0.14 0.16 units, or within any range encompassed bythe foregoing values, to provide the increased benefits to the shapememory wire discussed herein. It is thought that if the end point islower than 0.08, the mechanical conditioning may not impart the desiredphysical properties to the wire, as any dislocation or stress-inducedsecondary phase, such as martensite that is formed, may revert back tothe primary or parent phase, such as austenite. In contrast, if the endpoint is in excess of 0.14, the wire may potentially be strained by themechanical conditioning beyond its elastic deformation range to theextent that an undesirably large amount of plastic deformation mayresult.

An alternative characterization of the mechanical conditioning processof the present disclosure may be expressed as the application ofengineering stress to the material. For an application of force in atensile test as described herein, engineering stress is calculated usingthe following equation:

$\sigma_{e} = \frac{P}{A_{0}}$

where σ_(e) is the engineering stress, P is the force applied, and A₀ isthe cross-sectional area of the material before application of force. Inengineering stress terms, the range for σ_(e) may be from as little as700 MPa, 900 MPa or 1100 MPa to as much as 1350 MPa, 1450 MPa or 1600MPa, or within any range encompassed by the foregoing values, to providethe increased benefits to the shape memory wire discussed herein.

However, it is contemplated that force may be applied to the wire orwire product using alternate stress loading regimes, such as via methodsother than a tensile test, that may be more appropriate to the geometryof a particular wire product.

The temperature should be maintained below the martensitic deformationtemperature, M_(d), during the application of force to the wire in themechanical conditioning process. If the temperature exceeds themartensitic deformation temperature, the bulk of the shape memory wirematerial will not transform to martensite upon loading, because anyplastic deformation will occur in the austenite phase, and therefore thelocalized phase transformation mechanism would not occur. The entiretyof the wire material will remain in a plastically deformed austenitestate, with the austenite or primary phase containing significantplasticity and little retained martensite phase.

Referring to FIG. 6, once the desired stress or strain is applied to thewire, the force is removed and the sample is allowed to freely recoverand, if the temperature is below the austenitic finish temperature, orif further recovery of the bulk material is required, heat Q is appliedto drive the temperature, T to sufficiently greater than the austeniticfinish temperature (i.e., T>A_(f)) for recovery of the bulk material,leaving the material in a state wherein defects are still isolated by adislocation stabilized secondary phase as discussed below.

The steps of applying a controlled engineering stress or strain andsubsequently removing the force to allow the sample to freely recovermay repeated as few as 1, 2 or 3 times or as many as 6, 8 or 10 times,for example, to increase the amount of dislocation stabilized secondaryphase within the wire material, as discussed in detail below. Thus,after one or more applications of load cycles and the attendant recoveryof applied strain ε_(c) the length of wire 10 is greater than itsoriginal length L_(o). More particularly, the length of wire 10 afterload conditioning is L_(o)+ps, where ps is the permanent set orisothermally non-recoverable strain resulting from plastic,pseudoplastic and other deformation mechanisms, as shown in FIG. 6. Someof this isothermally non-recoverable strain can be recovered in the bulkmaterial by slight heating of the material as discussed above.

As discussed in more detail below in Section III, this isothermallynon-recoverable strain is indicative of the amount or volume of the wirematerial that has been converted from the primary phase to the secondaryphase and, upon recovery of the wire material, remains stabilized in thesecondary material phase. These localized areas of secondary phasematerial isolate defects and inhibit crack propagation in the primaryphase material.

For example, as calculated in Examples 3-7 below, this isothermallynon-recoverable strain may be calculated by measuring the difference inwire length after load removal in a tensile test. Known tensile testdevices (including the test device used for the present WorkingExamples) collect wire length data as the test is conducted. This data,not presented herein, is used to generate the permanent set datapresented in the tables. This non-recoverable length, with the originallength subtracted therefrom, gives a positive value where isothermallynon-recoverable deformation has occurred (i.e., “permanent set”). Thisdifference can then be divided by the original length, the product ofwhich is a strain value representing the isothermally non-recoverablestrain. This amount arises from a residual volume of altered materialwithin the wire which has accommodated a given amount of strain notrecovered upon load removal. The observed isothermally non-recoverablestrain may be divided by the load plateau strain length, which isassociated with the forward transformation from parent austenite phaseto secondary, stress-induced, martensite phase, thereby providing aquantitative indication of the volume fraction of altered materialwithin the wire.

This volume fraction, referred to as “max. volume martensite %” inTables 3-7 below is calculated using the following formula:

$V_{m} = \frac{INRS}{LPSL}$

wherein V_(M) is the maximum volume fraction of secondary phase, INRS isthe isothermally non-recoverable strain and LPSL is the loading plateaustrain length.

The volume fraction sets an upper limit on the amount of wire materialthat has been converted to the secondary phase from the primary phaseand remains stable at the given test temperature after load removal.That is to say, the total volume of material represented by thenon-recoverable strain comprises secondary phase material, and may alsocomprise other non-primary phase material arising from plasticallydeformed primary phase material or other deformation phenomena.

It is counter-intuitive that the application of stress to a wire productat a level sufficient to initiate plastic deformation according to thepresent mechanical conditioning process could be beneficial for the useof that wire product in a medical device that utilizes the shape memoryor superelastic characteristic of the wire product. One of ordinaryskill in the art would consider a wire product made of a shape memorymaterial that has been subjected to a stress level sufficient to induceany amount of plastic deformation to be compromised in its shape memoryor superelastic characteristic and therefore unsuitable for use in amedical device in which this characteristic is desired.

II. DESCRIPTION OF MATERIAL PROPERTIES OF WIRE PRODUCTS MADE INACCORDANCE WITH THE PRESENT MANUFACTURING PROCESS

Wire products made of shape memory materials or alloys that have beensubjected to the mechanical conditioning process of the presentdisclosure exhibit several novel physical characteristics and/or novelcombinations of physical characteristics, including the following:

1. Isolation of Defects

Referring to FIGS. 7( a)-(b), shape memory wire 10 may have one or moredefects, such as internal defects 28 and/or external defects 30. Thesedefects may include extrinsic defects and/or intrinsic defects such asinclusions or porosity as discussed above, for example.

These defects are isolated in localized fields or areas of secondaryphase material by subjecting the wire to mechanical conditioning, asexemplified by the curve shown in FIG. 8. As discussed above, this maybe accomplished by applying an engineering stress (and concomitantengineering strain) so that at least some parts of wire 10 experienceplastic deformation. In an exemplary embodiment of the present process,however, nearly all of the strain may be recovered upon unloading (FIG.8).

Referring now to FIGS. 9-10( c), mechanical conditioning results inareas of dislocation stabilized B19′, R, and/or martensite, shown assecondary phase areas 26 in FIG. 10, forming proximate defects 28 inwire 10. The formation of the secondary phase areas around and/oradjacent defects 28 during mechanical conditioning helps to retardfatigue crack growth in subsequent cyclic loading in a directionemanating from defects 28, as it is known that cracks propagate moreslowly in B19′, R, and/or martensite than in austenite. However, thebulk of wire 10, where defects are not present, reverts back to theaustenite phase after mechanical conditioning, such that the overallwire still exhibits its shape memory or superelastic characteristicwhile at the same time having an enhanced degree of fatigue strength dueto the isolation of defects within the secondary phase material.

Referring still to FIGS. 9-10( c), wires 10, 10′ and 10″ are shown aftermechanical conditioning. As a result of mechanical conditioning inaccordance with the present process, areas of dislocation stabilizedsecondary material phase 26 formed proximate material defects stabilizethe defects. That is to say, while the bulk of the wire material revertsback to the primary phase from the secondary phase, localized areas ofsecondary phase material remain formed proximate material defects. Thisstabilization of the secondary phase areas 26 is advantageous in that ithelps to retard fatigue crack growth in subsequent cyclic loading, forexample, as a crack generally propagates more slowly in the secondary(i.e., martensite) phase than in the primary (i.e., austenite) phase.

Stabilization of secondary phase areas 26 is, at least in part, due toplastic deformation 27 comprising dislocations and/or dislocationnetworks. This plastic deformation acts to stabilize secondary phase 26after removal of the conditioning mechanical conditioning stress orstrain (when primary phase returns to the bulk of the wire material) andduring subsequent service. The defect-free portions of the wire materialmay have less plastic deformation, or may have substantially no plasticdeformation. Therefore, this defect-free material will revert back tothe primary phase more readily and completely than the localizedsecondary phase areas near the defects, which have experienced plasticdeformation.

As shown in FIGS. 10( a)-(c), the shape, size and/or spatialconfiguration of secondary phase 26 varies depending upon thecharacteristics of the defect proximate the localized secondary phasefield. In general, the secondary phase field will form around thehighest stress areas of the defect, and may not form at lower stressareas. This is because plastic deformation occurs most readily at thesite of stress concentrators during the mechanical conditioning process;the dislocation stabilized secondary phase areas, which include someplastic deformation 27, will form at these stress concentration pointseven though the primary phase portions of the wire are still within arelatively elastic or pseudoelastic (where pseudoelastic is defined aselasticity associated with primary to secondary phase transformation)deformation range. Primary phase material remains present between anypair of defects that are sufficiently far apart, such that theirisolation fields to not overlap.

For example, wire 10 shown in FIG. 10( a) has a secondary phase area 26extending around substantially the entirety of defect 28. The geometryof defect 28, as well as the direction application of force F,determines the overall shape of secondary phase area 26.

As shown in FIG. 10( b), wire 10′ has defects 28′ with stabilizedsecondary phase areas 26′ at the highest stress concentration areascreated by the application of force F. Plastic deformation 27 alsooccurs within secondary phase areas 26′ as discussed above. Some ofsecondary phase areas 26′ are adjacent one another and have overlappingboundaries, so that a defect 28′ and another nearby defect 28′ will beinfluence one another.

Similarly, wire 10″ shown in FIG. 10( c) has multiple defects 28″ withstress fields 26″ and plastic deformation 27. Again, the stress fields28″ form at the highest stress concentration points, which are afunction of the geometry of defects 28″ and the direction of applicationof force F (shown as a longitudinal force along the axis of wire 10″) aswell as the temperature of the material during force application.

In this manner, shape memory material wire subjected to the presentmechanical conditioning process exhibits an enhanced fatigue life andfatigue strain threshold. Moreover, a shape memory wire made inaccordance with the present process retains overall material propertiesconsistent with wire in the austenitic phase, while exhibitinginhibition of crack propagation at defect sites consistent with themartensitic phase.

2. Increased High-Cycle Fatigue Resistance

As a result of the isolation of defects and/or defect boundaries (i.e.the sites along the defect/primary phase boundary most susceptible tostress concentration and crack propagation) in a secondary phase area orfield, mechanical conditioning increases the fatigue life andfatigue-strain threshold of the shape memory wire. As discussed inSection III, wire conditioned in accordance with an embodiment of thepresent disclosure exhibited a gain in the fatigue strain limit at 100million (10⁸) cycles of greater than 25% (FIG. 11). Also, as shown inFIG. 14, conditioned wire demonstrated an upward strain shift of greaterthan 20% at a 10 million (10⁷) cycle life (i.e., 1.1% engineering strainversus 0.9% engineering strain). Further, eight samples of thisconditioned material survived more than 10⁹ cycles and were stillrunning at the time conclusion of Example I discussed below.

3. Increased Damage Tolerance and Low-Cycle Fatigue Resistance

As discussed in Section III at Example 2, wire conditioning inaccordance with an embodiment of the present disclosure demonstratedincreased tolerance of damage to the wire. Three specimens with focusedion beam (FIB)-milled sharp defects were tested at an alternating strainof 1% in the conditioned and non-conditioned states. As shown in FIG.15, Conditioned samples demonstrated a 50% increase in damage tolerancecompared with the non-conditioned samples.

4. High Recoverable Strain, Low Residual Strain

As shown in FIGS. 13( a)-(c), greater than 8% recoverable engineeringstrain was observed with zero residual strain and good plateau stressesat body temperature (i.e., 310 K) after mechanical conditioning.

This recoverable strain renders wire made in accordance with the presentdisclosure particularly suitable for certain medical deviceapplications. As mentioned above, this is a counter-intuitive result.Typically, wire made of a shape memory or superelastic material whichhas been subjected to forces sufficient to cause any plastic deformationin the wire material would be considered “damaged”, and thereforeunsuitable for use in any medical device application. The surprisingresult of the present process is that, under proper mechanicalconditioning parameters as discussed herein, wire subjected to suchforces is actually superior for medical device applications.

III. EXAMPLES

The following non-limiting Examples illustrate various features andcharacteristics of the present invention, which are not to be construedas limited thereto.

The examples offer analysis of the effect of mechanical conditioning infine (such as less than 250 μm diameter) Nitinol wire, and particularlyof the effect of stress riser or defect isolation.

Example 1 Fatigue Resistant Nitinol Intermetallic Wire

Nanocrystalline, nominally Ti-56 wt. % Ni Nitinol wire (“NiTi wire”) wasmanufactured to create a superelastic, precipitate free wire with amedian grain size of 50 nm. An exemplary process for creating such awire is described in U.S. patent application Ser. No. 12/563,062,entitled FATIGUE DAMAGE RESISTANT WIRE AND METHOD OF PRODUCTION THEREOF,filed Sep. 18, 2009, the disclosure of which is hereby expresslyincorporated by reference herein in its entirety.

The resulting wire was drawn through diamond drawing dies beginning at adiameter of 0.230 mm and ending at a diameter of 0.177 mm to yield aretained cold work level of about 40% cold work. The wire was thencontinuously annealed at a temperature of 773K-873K for less than 60seconds, to yield a 50 nm grain size as verified by TEM electronmicroscopy scanning. Specifically, field emission scanning electronmicroscopy or transmission electron microscopy (TEM) is used to gatheran image containing, for example, several hundred crystals or grainsexhibiting strong grain boundary contrast. Next, the image is convertedto a binary format suitable for particle measurement. Resolvable grainsare modeled with ellipsoids and subsequently measured digitally yieldingstatistics regarding the crystal or grain size, such as the averagesize, maximum size, and minimum size. The resulting average crystal sizeis taken to be the average crystal size for the material from which thesample was taken. Grain size verification is discussed in detail in U.S.patent application Ser. No. 12/563,062, entitled FATIGUE DAMAGERESISTANT WIRE AND METHOD OF PRODUCTION THEREOF, filed Sep. 18, 2009,incorporated by reference above.

After annealing, the wire of the present Example was subjected to cyclictensile testing and was determined to exhibit pseudoelasticity out togreater than 10% engineering strain. A first wire sample was preservedat this point in the process.

The remainder of the wire was then subjected to mechanical conditioning,as described in detail above, by loading the wire to about 12% axialengineering strain (i.e., 0.12 units), completely releasing the load,reloading to about 12% engineering strain, and once again completelyreleasing the load. A second wire sample was preserved after this pointin the process.

The first and second wire samples were subjected to rotary beam fatiguetesting. Referring now to FIG. 11, the first, non-mechanicallyconditioned sample generated data curve 100 exhibiting a 100 M cycleengineering strain limit of about 0.85% at N=10 data points, shown byright most data point 100′ of the curve 100. The second, mechanicallyconditioned sample generated data curve 102 exhibiting a 100 M cycleengineering strain limit of about 1.1% at N=10 data points, exemplifyinga greater than 25% gain in the fatigue strain limit at 100 M cycles,shown by right most data point 102′ of curve 102.

Example 2 Mechanical Conditioning of Superelastic NiTi Wire for ImprovedFatigue Resistance

In this example, the effects of mechanical overload conditioning ofsuperelastic wire and the possibility of increased fatigue damageresistance associated with near-defect, plasticity-locked phasetransformation were investigated. In thin wires, where plane stressdominates, it is expected that sufficient loading will result in phasetransformation near the largest or shape-conducive crack-like defects,such as constituent inclusion particles, before conversion of the bulk,defect-free material.

1. Experimental Technique

Samples for this Example were subjected to a total engineering straindeparture, measured by crosshead extension, of about 11.5%. Conditioningwas applied by approaching the martensitic yield point at 295 K usingstrain-rate-controlled loading in order induce some dislocation lockingof stress-transformed material in the vicinity of stress concentrators.Referring now to FIG. 8, the conditioning cycle comprises astrain-controlled ramp to a stress level of 1240 MPa engineering stress,resulting in an engineering strain of about 11.5%, followed by a 3second hold, and finishing with a strain-controlled ramp to zero load.

In order to prepare samples for this Example, Nitinol wire with an ingotaustenite start temperature, AS, of 243 K, having Ti-56 wt. % Ni wasrepetitively drawn and annealed from a diameter of 2 mm to a diameter of177 μm in accordance with the process described above. At this stage,wires were continuously annealed at 770 to 800 K. Final cold working wascompleted using diamond dies to draw round wire with a diameter of 150μm prior to continuous, reel-to-reel annealing at 750 to 780 K underconstant engineering stress for less than 60 seconds to effect linearshape setting. The final wire comprised a room-temperature-superelasticNitinol wire with an active austenitic finish temperature, A_(f), of 280K and an approximately 120 nm thick, dark brown oxide layer similar tothat disclosed in an article by the present inventor entitled“Structure-Property Relationships in Conventional and NanocrystallineNiTi Intermetallic Alloy Wire” and published in the Journal of MaterialsEngineering and Performance, February 2008, ISSN 1544-1024, pgs. 1-6,the entire disclosure of which is hereby expressly incorporated byreference herein.

Focused ion beam (FIB)-milled sharp defects 202 (“FSD”) were then milledinto material 200 of each sample in order to act as preferential sitesof incipient fatigue crack formation and to facilitate user-defineddamage localization and monitoring. An FEI dual-beam (Nova 200 NanoLab)focused ion beam (FIB) with in situ scanning electron microscopy (SEM)was used to simultaneously monitor samples by SEM during the FIB millingprocess. A 30 keV Ga+ ion beam was used to precisely mill transversedefects into wire specimens at a 0.50 nA beam current. Defects 202 wereof consistent dimension measuring 10 μm transverse length by 3 μm radialdepth by 0.5 μm axial surface width, an example of which is shown inFIGS. 12( a)-(e). Cue lines 204 (FIG. 12( a)) were milled into the oxidesurface at a depth of about 50 nm on either side of each sharp defect inorder to enhance optical detection for accurate placement in fatiguetest gages after removal from the SEM chamber. As shown in FIGS. 12(a)-(e), the cue lines were of sufficient depth to create a visuallydetectible gradient associated with the reduced oxide thickness, whileshallow enough to minimize undesirable mechanical impact.

Electron microscopy of fracture surfaces was carried out using a Hitachi54800 field emission SEM (FE-SEM) operated at 10 to 20 kV. Transmissionelectron microscopy samples were extracted and prepared using theFIB/SEM dual beam equipment previously mentioned with an in situ samplemanipulator for thin foil removal and transport to TEM grids. Additionaldetails regarding this method can be found in the article by the presentinventor, which is incorporated by reference herein above, namely thearticle entitled “Structure-Property Relationships in Conventional andNanocrystalline NiTi Intermetallic Alloy Wire” and published in theJournal of Materials Engineering and Performance, February 2008, ISSN1544-1024, pgs. 1-6. TEM imaging and diffraction experiments werecarried out on a 200 kV machine equipped with a LaB₆ emitter (Tecnai 20,FEI Company, Oregon).

Cyclic and monotonic uniaxial tensile properties were measured at anambient temperature of 295 K at a strain rate of 10⁻³ s⁻¹ using anInstron Model 5565 Tensile Test Bench equipped with pneumatic, smoothface grips. Six hundred grit emery-cloth was used to reducegrip-specimen interface slip. Elevated temperature testing was completedon an equivalent tensile bench fitted with an environmental chambercapable of maintaining a temperature of 310±0.5 K.

Fatigue behavior was characterized using rotary beam fatigue testequipment manufactured by Positool, Inc., at a test rate of 60 s⁻¹ inambient 298 K air. The test rate was chosen at a rate significantlyhigher than physiological loading frequencies to promote expediency.Data have recently been presented by Robertson and Ritchie, in anarticle entitled “In vitro fatiguecrack growth and fracture toughnessbehavior of thin-walled superelastic Nitinol tube for endovascularstents: A basis for defining the effect of crack-like defects”,published in Biomaterials 28, 2007, pgs. 700-709, suggesting that highrate testing may well estimate in vivo fatigue failure lifetimes.

As shown in FIGS. 13( a)-13(c), specimens from each group,non-conditioned (NC) and conditioned (C), were tested at alternatingengineering strain (½ peak-to-peak amplitude) levels ranging from 0.8 to1.6% to a maximum of about 10⁹ cycles or 200 days test time. Further,specimens with FSD 202 were tested at 1% engineering strain before andafter conditioning. The FSD zone was located at the apex of the fatiguebend by optical positioning using the cue marks 204 as guides.

2. Results

The resulting tensile data is shown graphically in FIGS. 13( a)-(c). Asnoted above, the conditioning cycle comprised a strain-controlled rampto a stress level of 1240 MPa engineering stress followed by a 3 secondhold, finishing with a strain-controlled ramp to zero load. Thisconditioning cycle generated data curve 220, shown in FIGS. 13( a)-(c).The total engineering strain departure for this cycle, measured bycrosshead extension, was 11.5%. Conditioning initially resulted in 0.3%residual strain comprising both plastic and pseudo-plastic straincontributions.

The martensite to austenite reversion plateau stress associated withunloading was significantly reduced during unloading from theconditioning cycle, but was observed to elevate in subsequent testing to8% engineering strain. Some of this effect can be accounted for bystrain rate differences: the conditioning cycle was run at asignificantly higher strain rate than the 8% test cycles. High strainrates can cause heating during loading and cooling during unloadingresulting in increasing stress hysteresis. Further testing of a C sampleat body temperature (310 K) generated data curve 226 depicted on FIGS.13( a)-(c) by circular marks 226′, showing elevation of the unloadingplateau stress to levels greater than an NC sample at 295 K, shown asdata curve 222 with square-shaped marks 222. This result was consistentwith known test temperature-plateau stress relationships.

The conditioned sample at 295K, shown as curve 224 on FIGS. 13( a)-(c)with triangle-shaped marks 224′, exhibited a downward shift in theunloading plateau stress. This can be attributed to plastic deformation,some of which may be directly beneficial to resistance againstsubsequent fatigue crack growth. The lack of significant shift in thestrain length of the plateaus indicates that plastic deformation to theoverall microstructure was minimal during overload conditioning.

FIG. 14 illustrates the observed differences in fatigue performance forconditioned (C) wire specimens, shown as data curve 230 withtriangular-shaped marks 230′, versus non-conditioned (NC) wirespecimens, shown as data curve 232 with triangular-shaped marks 232′.The conditioning resulted in an upward strain shift of greater than 20%at the 10⁷ cycle life (i.e., 1.1% engineering strain versus 0.9%engineering strain). Eight samples of the conditioned material survivedmore than 10⁹ cycles and were still running at the time conclusion ofthe experiment.

As shown in FIG. 15, three FSD specimens in each of the NC and C stateswere tested at an alternating strain of 1%. In this case, the FSD-Cgroup generated data bar graph 242, showing an average of 21,228 cyclesto failure with a margin of error indicated at the top of the bar. TheFSD-NC group generated data bar graph 240, showing an average of 14,196cycles to failure with a margin of error indicated at the top of thebar. Thus, the conditioned wire samples outperformed the non-conditionedsamples by 50%. All FSD samples failed considerably before the non-FSDsamples; this is attributable to the geometry of the FIB-milled defects,which were purposefully milled larger and sharper than the 2-6 μminclusion particles typically found at fatigue failure sites in thisgrade of Nitinol wire in order to direct site-specific, locatablefailure for study.

A microstructurally distinct region resulting from the mechanicalconditioning was found within an approximately 500 nm radius of theapproximately 10 nm width FSD crack root. FIG. 16 shows the results ofTEM work performed to help elucidate mechanisms giving rise tomechanical property changes associated with the mechanical conditioning.The selected area diffraction patterns 250, 252 in FIG. 16, outside ofand within the distinct FSD concentration zone respectively, revealsignificant differences in contributing bright field contrast signal. Atypical polycrystalline, B2 pattern 250 was observed at approximately 1μm from the crack tip, while the selected area diffraction patternadjacent to the root, shown as 252, revealed what appears to besuperimposed diffuse rings, B2 polycrystalline reflections, as well assome evidence of ½(110) reflections associated with the B19′ martensiticphase. Also evident is a significant increase in dislocation density andassociated contrast.

The diffuse (110) rings observed within the FSD zone, shown in the leftand upper-right insets of FIG. 16, may be related to partialamorphization and/or due to (110) reflection splitting and the presenceof ½(110) reflections associated with a mixed B2-B19′ structure.

The presence of plasticity in the FSD region may also contribute toincreased fatigue performance due to residual stresses which offset theeffective crack-opening stress intensity range. The reduction in theoperating stress intensity range can serve to increase the effectiveΔK_(th) fatigue threshold (defined above) thereby elevating the strainload level required to initiate or maintain crack growth. The mixedmicrostructure may also promote crack front tortuosity therebyprecluding crack arrest associated with roughness-induced closure atnear-threshold crack growth conditions.

The narrowest ductile striations were observed in conditioned samplesnear the FSD incipient crack front. High resolution SEM (HR-SEM)analysis of fatigue failure sites in NC and C specimens was completedand the stress intensity was estimated based on the probable crack frontlocation at the examined site using assumptions of a semi-ellipticalcrack in an infinite cylinder. Referring to FIG. 17, a crack growth rateplot as a function of the estimated stress intensity factor (not takinginto account crack closure effects) is shown, with square-shaped marks260 indicating data on non-conditioned material and triangle-shapedmarks 262 indicating conditioned material. The difference between thetwo data sets may suggest martensitic growth rate inhibition.

In this example, it has been demonstrated that mechanical conditioningof superelastic NiTi wire results in improved fatigue performance, whilemaintaining good mechanical properties. In addition, greater than 8%recoverable engineering strain was observed with zero residual strainand good plateau stresses at body temperature after mechanicalconditioning. Further, an increase in the strain fatigue limit ofgreater than 20% at 10⁷ cycles is observed in conditioned versusnon-conditioned wire with an observed increase in low cycle life of 50%.The tensile overload conditioning treatment also resulted in amixed-phase microstructure in the vicinity of stress concentrators thatcomprises increased dislocation density and possible plasticity-inducedor roughness-induced crack closure.

Introduction to Examples 3-7

For examples 3-7, various wire materials were tested in a similar mannerfor comparison to one another. FIGS. 18( a)-(d) show results for wirematerials from each of Examples 3-7, with each Figure representing adifferent strain condition (as indicated on each respective Figure).Table 1, below, indexes the Example materials:

TABLE 1 Index of Example Materials for Examples 3-6 Austenitic NominalActual Shape set Surface Finish Nitinol Composition Diameter temperatureFinish Temperature Example type (weight %) (mm) (K) Description Af (K) 3NiTi #1 56Ni—Ti 0.151 740-800 Oxide 280 4 NiTi #2 55.8Ni—0.25Cr—Ti 0.269740-800 Oxide 283 5 NiTi #4 55.8Ni—Ti 0.302 740-800 Oxide 298 6 NiTi #156Ni—Ti 0.076 740-800 Etched 288 7 NiTi #1 56Ni—Ti 0.638 740-800Polished 291

FIGS. 19( a)-(e) also show results for wire materials from each ofExamples 3-7, with each Figure representing a stress-strain curve for acorresponding Example. Table 2, below, indexes the mechanicalconditioning regimes applied to each wire product shown in FIGS. 19(a)-(e). The “regime” number corresponds to a given curve on each figure,as indicated by the corresponding number on the legend at the right sideof each of FIGS. 19( a)-(e), where, the digit preceding the decimalpoint in X.X refers to the Example (e.g. 1, 2, 3, 4, and 5 correspond toExamples 3, 4, 5, 6, and 7 respectively), and the digit following thedecimal point corresponds to regimes 1, 3, 4, 6, and 7 given below inTable 2.

TABLE 2 Index of Conditioning Regimes for Examples 3-6 Regime (See alsoRamp rate Hold stress (MPa) legends at right (engineering (See alsoX-axes of FIGS. 19(a)-(e)) strain/min) in FIGS. 18(a)-(d)) 1 0.10 700 30.10 1100 4 0.10 1240 6 0.10 1400 7 0.10 1500

Results shown in Tables 3-7 show data used to create FIGS. 19( a)-19(e)respectively. Thus, Tables 3-7 correspond with Examples 3-7,respectively.

Example 3 Mechanical Conditioning of Superelastic NiTi Wire for ImprovedFatigue Resistance

In this example, the effects of mechanical overload conditioning ofsuperelastic wire and the possibility of increased fatigue damageresistance associated with near-defect, plasticity-locked phasetransformation were further investigated over a broader range of loadsas compared to Example 2.

1. Experimental Technique

Samples for this Example were subjected to a total engineering straindeparture, measured by crosshead extension, ranging from about 8% to12.5% Conditioning was applied by approaching the martensitic yieldpoint at 295 K using strain-rate-controlled loading in order induce somedislocation locking of stress-transformed material in the vicinity ofstress concentrators. Referring now to FIG. 19( a), the conditioningcycle comprises a strain-controlled ramp to five stress levels of 700,1100, 1240, 1400, and 1500 MPa engineering stress, resulting in anengineering strain of about 8.3%, 9.8%, 10.3%, 11.1% and 12.2%respectively, followed by a 3 second hold, finishing with astrain-controlled ramp to zero load.

In order to prepare samples for this Example, Nitinol wire with an ingotaustenite start temperature, A_(s), of 243 K, having Ti-56 wt. % Ni wasrepetitively drawn and annealed from a diameter of 2 mm to a diameter of201 μm in accordance with the process described above. At this stage,wires were continuously annealed at 950 to 1000 K. Final cold workingwas completed using diamond dies to draw round wire with a diameter of151 μm prior to continuous, reel-to-reel annealing at 750 to 780 K underconstant engineering stress for 40 to 80 seconds to effect linear shapesetting. The final wire comprised a room-temperature-superelasticNitinol wire with an active austenitic finish temperature, A_(f), of 280K and an approximately 120 nm thick, dark brown oxide layer similar tothat disclosed in an article by the present inventor entitled“Structure-Property Relationships in Conventional and NanocrystallineNiTi Intermetallic Alloy Wire” and published in the Journal of MaterialsEngineering and Performance, February 2008, ISSN 1544-1024, pgs. 1-6,which is incorporated by reference above.

Cyclic and monotonic uniaxial tensile properties were measured at anambient temperature of 295 K at a strain rate of 10⁻³ s⁻¹ using anInstron Model 5565 Tensile Test Bench equipped with pneumatic, smoothface grips. Six hundred grit emery-cloth was used to reducegrip-specimen interface slip.

Fatigue behavior was characterized using rotary beam fatigue testequipment manufactured by Positool, Inc., at a test rate of 60 s⁻¹ inambient 298 K air. The test rate was chosen at a rate significantlyhigher than physiological loading frequencies to promote expediency.

As shown in FIGS. 18( a)-(d), three specimens from each group at eachconditioning cycle, ranging from 0 MPa which indicates non-conditionedwire to 1500 MPa which indicates the maximum conditioning load used,were tested at alternating engineering strain (½ peak-to-peak amplitude)levels ranging from 0.80 to 1.25% strain to a maximum of about 10⁶cycles. The total samples tested for each conditioning load regime foreach sample was 12 resulting in a total of 72 fatigue samples tested forthis portion of the study. Samples which did not fracture after 10⁶cycles were stopped and recorded.

2. Results

The resulting tensile data is shown graphically in FIGS. 18( a)-(d) asline 1 on each curve. As noted above, the conditioning cycle comprised astrain-controlled ramp to a stress level of 0, 700, 1100, 1240, 1400 or1500 MPa engineering stress, as indicated along the horizontal axis ofthe plot, followed by a 3 second hold, finishing with astrain-controlled ramp to zero load.

This conditioning cycle generated data curves 1.1, 1.3, 1.4, 1.6 and 1.7shown in FIG. 19( a). The total engineering strain departure forrespective cycles, measured by crosshead extension, was about 0%, 8.3%,9.8%, 10.3%, 11.1% and 12.2% for increasing load levels respectively.Conditioning resulted in residual strains (i.e. isothermallynon-recoverable strains) of respectively 0%, 0.19%, 0.20%, 0.24%, 0.17%,ad 7.3% respectively.

FIGS. 18 (a) to (d) illustrate the observed differences in fatigueperformance for non-conditioned (e.g. 0 load level on x-axis) andconditioned wire specimens (700, 1100, 1240, 1400, and 1500 MPa on thex-axis) The conditioning resulted in an upward cycle life shift of atleast 55% and 3000% at the 1.25% and 0.95% alternating strain testlevels respectively at a conditioning load level of 1240 MPa. An overallupward trend in lifetime for a given test strain level was observed forincreasing conditioning load through 1500 MPa. Most samples of thematerial conditioned at greater than 1240 MPa survived more than 10⁶cycles and were still running at the time of conclusion of theexperiment for test strain levels below 0.95%.

FIG. 19( a) illustrates the observed tensile behavior during loadconditioning of each sample. In each case, an upper bound of the maximumvolume of retained martensite was calculated as described above based onthe ratio of isothermally non-recoverable strain to the strain length ofthe load plateau. FIG. 20 illustrates the positive correlation betweenisothermally non-recoverable strain and conditioning load.Non-recoverable strain was less than 0.17% for all samples conditionedbelow 1400 MPa resulting in a max. volume of retained martensiteestimate of 3.7% for the same samples loaded below 1400 MPa.

In this Example, it has been demonstrated that mechanical conditioningof superelastic NiTi wire results in improved fatigue performance, whilemaintaining good mechanical properties. In addition, less than 3.7% ofthe matrix was left in the martensite phase after load removal forconditioning below 1400 MPa with a concomitant maximum isothermallynon-recoverable strain of 0.17%. Further, an increase in the strainfatigue life of greater than 3000% at 10⁶ cycles is observed in wireconditioned at 1240 MPa versus non-conditioned wire while maintaininggood elastic properties suitable for said medical device applications.

Example 4 Mechanical Conditioning of Superelastic NiTi Wire for ImprovedFatigue Resistance

In this Example, the effects of mechanical overload conditioning ofsuperelastic wire and the possibility of increased fatigue damageresistance associated with near-defect, plasticity-locked phasetransformation were further investigated over a broader range of loadsas compared to Example 2 using a high strength chromium doped tertiaryNitinol compound.

1. Experimental Technique

Samples for this Example were subjected to a total engineering straindeparture, measured by crosshead extension, ranging from about 7.7% to13.1% Conditioning was applied by approaching the martensitic yieldpoint at 295 K using strain-rate-controlled loading in order induce somedislocation locking of stress-transformed material in the vicinity ofstress concentrators. Referring now to FIG. 19( b), the conditioningcycle comprises a strain-controlled ramp to five stress levels of 0,700, 1100, 1240, 1400, and 1500 MPa engineering stress, resulting in anengineering strain of about 0%, 7.7%, 9.6%, 10.2%, 11.3%, and 13.1%respectively, followed by a 3 second hold, finishing with astrain-controlled ramp to zero load.

In order to prepare samples for this Example, Nitinol wire with an ingotaustenite start temperature, A_(s), of about 235 K, having Ti-55.8 wt. %Ni-0.25 wt % Cr was repetitively drawn and annealed from a diameter of 2mm to a diameter of 361 nm in accordance with the process describedabove. At this stage, wires were continuously annealed at 950 to 1000 K.Final cold working was completed using diamond dies to draw round wirewith a diameter of 269 nm prior to continuous, reel-to-reel annealing at750 to 780 K under constant engineering stress for 40 to 80 seconds toeffect linear shape setting. The final wire comprised aroom-temperature-superelastic Nitinol wire with an active austeniticfinish temperature, A_(f), of 283 K and an approximately 120 nm thick,dark brown oxide layer similar to that disclosed in an article by thepresent inventor entitled “Structure-Property Relationships inConventional and Nanocrystalline NiTi Intermetallic Alloy Wire” andpublished in the Journal of Materials Engineering and Performance,February 2008, ISSN 1544-1024, pgs. 1-6, which is incorporated byreference above.

Cyclic and monotonic uniaxial tensile properties were measured at anambient temperature of 295 K at a strain rate of 10⁻³ s⁻¹ using anInstron Model 5565 Tensile Test Bench equipped with pneumatic, smoothface grips. Six hundred grit emery-cloth was used to reducegrip-specimen interface slip.

Fatigue behavior was characterized using rotary beam fatigue testequipment manufactured by Positool, Inc., at a test rate of 60 s⁻¹ inambient 298 K air. The test rate was chosen at a rate significantlyhigher than physiological loading frequencies to promote expediency.

As shown in FIGS. 18 (a) to (d), three specimens from each group at eachconditioning cycle, ranging from 0 MPa which indicates non-conditionedwire to 1500 MPa which indicates the maximum conditioning load used,were tested at alternating engineering strain (½ peak-to-peak amplitude)levels ranging from 0.80 to 1.25% strain to a maximum of about 10⁶cycles. The total samples tested for each conditioning load regime foreach sample was 12 resulting in a total of 72 fatigue samples tested forthis portion of the study. Samples which did not fracture after 10⁶cycles were stopped and recorded.

2. Results

The resulting tensile data is shown graphically in FIG. 19( b). As notedabove, the conditioning cycle comprised a strain-controlled ramp to astress level of 0, 700, 1100, 1240, 1400 or 1500 MPa engineering stress,as indicated along the horizontal axis of the plot, followed by a 3second hold, finishing with a strain-controlled ramp to zero load. Thisconditioning cycle generated data curves 2.1, 2.3, 2.4, 2.6 and 2.7shown in FIG. 19( b). The total engineering strain departure forrespective cycles, measured by crosshead extension, was about 0%, 7.7%,9.6%, 10.2%, 11.3%, and 13.1% for increasing load levels respectively.Conditioning resulted in residual strains (i.e. isothermallynon-recoverable strains) of respectively 0%, 0.07%, 0.09%, 0.35%, 0.18%,and 2.23% respectively.

FIGS. 18 (a)-(d) illustrate the observed differences in fatigueperformance for non-conditioned (e.g. 0 load level on x-axis) andconditioned wire specimens (700, 1100, 1240, 1400, and 1500 MPa on thex-axis) The conditioning resulted in an upward cycle life shift of atleast 46% and 2500% at the 1.25% and 0.95% alternating strain testlevels respectively at a conditioning load level of 1240 MPa. An overallupward trend in lifetime for a given test strain level was observed forincreasing conditioning load through 1500 MPa. Most samples of thematerial conditioned at greater than 1240 MPa survived more than 10⁶cycles and were still running at the time of conclusion of theexperiment for test strain levels below 0.95%.

FIG. 19( b) illustrates the observed tensile behavior during loadconditioning of each sample. In each case, an upper bound of the maximumvolume of retained martensite was calculated as described above based onthe ratio of isothermally non-recoverable strain to the strain length ofthe load plateau. FIG. 20 illustrates the positive correlation betweenisothermally non-recoverable strain and conditioning load.Non-recoverable strain was less than 0.35% for all samples conditionedbelow 1400 MPa resulting in a max volume of retained martensite estimateof 5.9% for the same samples loaded below 1400 MPa.

In this Example, it has been demonstrated that mechanical conditioningof superelastic NiTi wire results in improved fatigue performance, whilemaintaining good mechanical properties. In addition, less than 5.9% ofthe matrix was left in the martensite phase after load removal forconditioning below 1400 MPa with a concomitant maximum isothermallynon-recoverable strain of 0.35%. Further, an increase in the strainfatigue life of greater than 2500% at 10⁶ cycles is observed in wireconditioned at 1240 MPa versus non-conditioned wire while maintaininggood elastic properties suitable for said medical device applications.

Example 5 Mechanical Conditioning of Superelastic NiTi Wire for ImprovedFatigue Resistance

In this Example, the effects of mechanical overload conditioning ofsuperelastic wire and the possibility of increased fatigue damageresistance associated with near-defect, plasticity-locked phasetransformation were further investigated over a broader range of loadsas compared to Example 2 using a Nitinol with warmer transformationtemperatures as compared to Examples 2 and 3.

1. Experimental Technique

Samples for this Example were subjected to a total engineering straindeparture, measured by crosshead extension, ranging from about 8.3% to12.8% Conditioning was applied by approaching the martensitic yieldpoint at 295 K using strain-rate-controlled loading in order induce somedislocation locking of stress-transformed material in the vicinity ofstress concentrators. Referring now to FIG. 19 (c), the conditioningcycle comprises a strain-controlled ramp to five stress levels of 0,700, 1100, 1240, and 1400 MPa engineering stress, resulting in anengineering strain of about 0%, 8.3%, 10%, 10.8%, and 12.8%respectively, followed by a 3 second hold, finishing with astrain-controlled ramp to zero load.

In order to prepare samples for this Example, Nitinol wire with an ingotaustenite start temperature, A_(s), of about 255 K, having Ti-55.8 wt. %Ni was repetitively drawn and annealed from a diameter of 2 mm to adiameter of 380 μm in accordance with the process described above. Atthis stage, wires were continuously annealed at 950 to 1000 K. Finalcold working was completed using diamond dies to draw round wire with adiameter of 302 μm prior to continuous, reel-to-reel annealing at 750 to780 K under constant engineering stress for 40 to 80 seconds to effectlinear shape setting. The final wire comprised aroom-temperature-superelastic Nitinol wire with an active austeniticfinish temperature, A_(f), of 298 K and an approximately 120 nm thick,dark brown oxide layer similar to that disclosed in an article by thepresent inventor entitled “Structure-Property Relationships inConventional and Nanocrystalline NiTi Intermetallic Alloy Wire” andpublished in the Journal of Materials Engineering and Performance,February 2008, ISSN 1544-1024, pgs. 1-6, which is incorporated byreference above.

Cyclic and monotonic uniaxial tensile properties were measured at anambient temperature of 295 K at a strain rate of 10⁻³ s⁻¹ using anInstron Model 5565 Tensile Test Bench equipped with pneumatic, smoothface grips. Six hundred grit emery-cloth was used to reducegrip-specimen interface slip.

Fatigue behavior was characterized using rotary beam fatigue testequipment manufactured by Positool, Inc., at a test rate of 60 s⁻¹ inambient 298 K air. The test rate was chosen at a rate significantlyhigher than physiological loading frequencies to promote expediency.

As shown in FIGS. 18 (a) to (d), three specimens from each group at eachconditioning cycle, ranging from 0 MPa which indicates non-conditionedwire to 1400 MPa which indicates the maximum conditioning load used,were tested at alternating engineering strain (½ peak-to-peak amplitude)levels ranging from 0.80 to 1.25% strain to a maximum of about 10⁶cycles. The total samples tested for each conditioning load regime foreach sample was 12 resulting in a total of 60 fatigue samples tested forthis portion of the study. Samples which did not fracture after 10⁶cycles were stopped and recorded.

2. Results

The resulting tensile data is shown graphically in FIG. 19 (c). As notedabove, the conditioning cycle comprised a strain-controlled ramp to astress level of 0, 700, 1100, 1240, or 1400 MPa engineering stress, asindicated along the horizontal axis of the plot, followed by a 3 secondhold, finishing with a strain-controlled ramp to zero load. Thisconditioning cycle generated curves 3.1, 3.3, 3.4, and 3.6 shown in FIG.19( c). The total engineering strain departure for respective cycles,measured by crosshead extension, was about 0%, 8.3%, 10%, 10.8%, and12.8% for increasing load levels respectively. Conditioning resulted inresidual strains (i.e. isothermally non-recoverable strains) ofrespectively 0%, 0.03%, 0.13%, 1.1%, and 7.31% respectively.

FIGS. 18 (a) to (d) illustrate the observed differences in fatigueperformance for non-conditioned (e.g. 0 load level on x-axis) andconditioned wire specimens (700, 1100, 1240, and 1400 MPa on the x-axis)The conditioning resulted in an upward cycle life shift of at least 116%and 2400% at the 1.25% and 0.95% alternating strain test levelsrespectively at a conditioning load level of 1240 MPa. An overall upwardtrend in lifetime for a given test strain level was observed forincreasing conditioning load through 1400 MPa. Most samples of thematerial conditioned at greater than 1240 MPa survived more than 10⁶cycles and were still running at the time of conclusion of theexperiment for test strain levels below 0.95%.

FIG. 19( c) illustrates the observed tensile behavior during loadconditioning of each sample. In each case, an upper bound of the maximumvolume of retained martensite was calculated as described above based onthe ratio of isothermally non-recoverable strain to the strain length ofthe load plateau. FIG. 20 illustrates the positive correlation betweenisothermally non-recoverable strain and conditioning load.Non-recoverable strain was less than about 1% for all samplesconditioned below 1240 MPa resulting in a max. volume of retainedmartensite estimate of about 17% for the same samples loaded below 1240MPa.

In this Example, it has been demonstrated that mechanical conditioningof superelastic NiTi wire results in improved fatigue performance, whilemaintaining good mechanical properties. In addition, less than about 17%of the matrix was left in the martensite phase after load removal forconditioning below 1240 MPa with a concomitant maximum isothermallynon-recoverable strain of about 1%. Further, an increase in the strainfatigue life of greater than 2400% at 10⁶ cycles is observed in wireconditioned at 1240 MPa versus non-conditioned wire while maintaininggood elastic properties suitable for said medical device applications.

Example 6 Mechanical Conditioning of Superelastic NiTi Wire with anEtched Surface Finish for Improved Fatigue Resistance

In this Example, the effects of mechanical overload conditioning ofsuperelastic wire and the possibility of increased fatigue damageresistance associated with near-defect, plasticity-locked phasetransformation were further investigated over a broader range of loadsas compared to Example 2 and in a finer diameter using a Nitinol with anetched surface finish comprising a substantially oxide-free surface.

1. Experimental Technique

Samples for this Example were subjected to a total engineering straindeparture, measured by crosshead extension, ranging from about 7.8 to11.9% Conditioning was applied by approaching the martensitic yieldpoint at 295 K using strain-rate-controlled loading in order induce somedislocation locking of stress-transformed material in the vicinity ofstress concentrators. Referring now to FIG. 19 (d), the conditioningcycle comprises a strain-controlled ramp to five stress levels of 0,700, 1100, 1240, 1400 and 1500 MPa engineering stress, resulting in anengineering strain of about 0%, 7.8%, 9.5%, 10%, 10.8%, and 11.9%respectively, followed by a 3 second hold, finishing with astrain-controlled ramp to zero load.

In order to prepare samples for this Example, Nitinol wire with an ingotaustenite start temperature, A_(s), of about 246 K, having Ti-56 wt. %Ni was repetitively drawn and annealed from a diameter of 2 mm to adiameter of 102 nm in accordance with the process described above. Atthis stage, wires were continuously annealed at 950 to 1000 K. Finalcold working was completed using diamond dies to draw round wire with adiameter of 76 nm prior to continuous, reel-to-reel annealing at 750 to780 K under constant engineering stress for 40 to 80 seconds to effectlinear shape setting. The final wire comprised aroom-temperature-superelastic Nitinol wire with an active austeniticfinish temperature, A_(f), of 288 K and an etched, substantiallyoxide-free surface finish.

Cyclic and monotonic uniaxial tensile properties were measured at anambient temperature of 295 K at a strain rate of 10⁻³ s⁻¹ using anInstron Model 5565 Tensile Test Bench equipped with pneumatic, smoothface grips. Six hundred grit emery-cloth was used to reducegrip-specimen interface slip.

Fatigue behavior was characterized using rotary beam fatigue testequipment manufactured by Positool, Inc., at a test rate of 60 s⁻¹ inambient 298 K air. The test rate was chosen at a rate significantlyhigher than physiological loading frequencies to promote expediency.

As shown in FIGS. 18 (a) to (d), three specimens from each group at eachconditioning cycle, ranging from 0 MPa which indicates non-conditionedwire to 1500 MPa which indicates the maximum conditioning load used,were tested at alternating engineering strain (½ peak-to-peak amplitude)levels ranging from 0.80 to 1.25% strain to a maximum of about 10⁶cycles. The total samples tested for each conditioning load regime foreach sample was 12 resulting in a total of 72 fatigue samples tested forthis portion of the study. Samples which did not fracture after 10⁶cycles were stopped and recorded.

2. Results

The resulting tensile data is shown graphically in FIG. 19 (d). As notedabove, the conditioning cycle comprised a strain-controlled ramp to astress level of 0, 700, 1100, 1240, 1400 or 1500 MPa engineering stress,as indicated along the horizontal axis of the plot, followed by a 3second hold, finishing with a strain-controlled ramp to zero load. Thisconditioning cycle generated curves 4.1, 4.3, 4.4, 4.6 and 4.7 shown inFIG. 19( d). The total engineering strain departure for respectivecycles, measured by crosshead extension, was about 0%, 7.8%, 9.5%, 10%,10.8%, and 11.9% for increasing load levels respectively. Conditioningresulted in residual strains (i.e. isothermally non-recoverable strains)of respectively 0%, 0.09%, 0.14%, 0.29%, 0.26% and 7.2% respectively.

FIGS. 18 (a) to (d) illustrate the observed differences in fatigueperformance for non-conditioned (e.g. 0 load level on x-axis) andconditioned wire specimens (700, 1100, 1240, 1400 and 1500 MPa on thex-axis) The conditioning resulted in an upward cycle life shift of atleast 7.2% and 2600% at the 1.25% and 0.95% alternating strain testlevels respectively at a conditioning load level of 1240 MPa. An overallupward trend in lifetime for a given test strain level was observed forincreasing conditioning load through 1500 MPa. Most samples of thematerial conditioned at greater than 1240 MPa survived more than 10⁶cycles and were still running at the time of conclusion of theexperiment for test strain levels below 0.95%.

FIG. 19( d) illustrates the observed tensile behavior during loadconditioning of each sample. In each case, an upper bound of the maximumvolume of retained martensite was calculated as described above based onthe ratio of isothermally non-recoverable strain to the strain length ofthe load plateau. FIG. 20 illustrates the positive correlation betweenisothermally non-recoverable strain and conditioning load.Non-recoverable strain was less than about 0.29% for all samplesconditioned below 1240 MPa resulting in a max. volume of retainedmartensite estimate of about 4.7% for the same samples loaded below 1400MPa.

In this Example, it has been demonstrated that mechanical conditioningof superelastic NiTi wire results in improved fatigue performance, whilemaintaining good mechanical properties. In addition, less than about4.7% of the matrix was left in the martensite phase after load removalfor conditioning below 1400 MPa with a concomitant maximum isothermallynon-recoverable strain of about 0.29%. Further, an increase in thestrain fatigue life of greater than 2600% at 10⁶ cycles is observed inwire conditioned at 1240 MPa versus non-conditioned wire whilemaintaining good elastic properties suitable for said medical deviceapplications.

Example 7 Mechanical Conditioning of Superelastic NiTi Wire with aPolished Surface Finish for Improved Fatigue Resistance

In this Example, the effects of mechanical overload conditioning ofsuperelastic wire and the possibility of increased fatigue damageresistance associated with near-defect, plasticity-locked phasetransformation were further investigated over a broader range of loadsas compared to Example 2 and in a larger diameter using a Nitinol withan etched and mechanically polished surface finish comprising asubstantially oxide-free surface.

1. Experimental Technique

Samples for this Example were subjected to a total engineering straindeparture, measured by crosshead extension, ranging from about 7.5 to13.1% Conditioning was applied by approaching the martensitic yieldpoint at 295 K using strain-rate-controlled loading in order induce somedislocation locking of stress-transformed material in the vicinity ofstress concentrators. Referring now to FIG. 19 (d), the conditioningcycle comprises a strain-controlled ramp to five stress levels of 0,700, 1100, 1240, 1400 and 1500 MPa engineering stress, resulting in anengineering strain of about 0%, 7.5%, 9.6%, 10.3%, 11.6%, and 13.1%respectively, followed by a 3 second hold, finishing with astrain-controlled ramp to zero load.

In order to prepare samples for this Example, Nitinol wire with an ingotaustenite start temperature, A_(s), of about 249 K, having Ti-56 wt. %Ni was repetitively drawn and annealed from a diameter of 2 mm to adiameter of 813 μm in accordance with the process described above. Atthis stage, wires were continuously annealed at 950 to 1000 K. Finalcold working was completed using diamond dies to draw round wire with adiameter of 638 μm prior to continuous, reel-to-reel annealing at 750 to780 K under constant engineering stress for 60 to 150 seconds to effectlinear shape setting. The final wire comprised aroom-temperature-superelastic Nitinol wire with an active austeniticfinish temperature, A_(f), of 291 K and an etched, substantiallyoxide-free and mechanically polished surface finish.

Cyclic and monotonic uniaxial tensile properties were measured at anambient temperature of 295 K at a strain rate of 10⁻³ s⁻¹ using anInstron Model 5565 Tensile Test Bench equipped with pneumatic, smoothface grips. Six hundred grit emery-cloth was used to reducegrip-specimen interface slip.

Fatigue behavior was characterized using rotary beam fatigue testequipment manufactured by Positool, Inc., at a test rate of 60 s⁻¹ inambient 298 K air. The test rate was chosen at a rate significantlyhigher than physiological loading frequencies to promote expediency.

As shown in FIGS. 18 (a) to (d), three specimens from each group at eachconditioning cycle, ranging from 0 MPa which indicates non-conditionedwire to 1500 MPa which indicates the maximum conditioning load used,were tested at alternating engineering strain (½ peak-to-peak amplitude)levels ranging from 0.80 to 1.25% strain to a maximum of about 10⁶cycles. The total samples tested for each conditioning load regime foreach sample was 12 resulting in a total of 72 fatigue samples tested forthis portion of the study. Samples which did not fracture after 10⁶cycles were stopped and recorded.

2. Results

The resulting tensile data is shown graphically in FIG. 19 (e). As notedabove, the conditioning cycle comprised a strain-controlled ramp to astress level of 0, 700, 1100, 1240, 1400 or 1500 MPa engineering stress,as indicated along the horizontal axis of the plot, followed by a 3second hold, finishing with a strain-controlled ramp to zero load. Thisconditioning cycle generated data curves 5.1, 5.3, 5.4, 5.6 and 5.7shown in FIG. 19( e). The total engineering strain departure forrespective cycles, measured by crosshead extension, was about 0%, 7.5%,9.6%, 10.3%, 11.6%, and 13.1% for increasing load levels respectively.Conditioning resulted in residual strains (i.e. isothermallynon-recoverable strains) of respectively 0%, 0.21%, 0.40%, 0.61%, 2.17%,and 7.84% respectively.

FIGS. 18 (a) to (d) illustrate the observed differences in fatigueperformance for non-conditioned (e.g. 0 load level on x-axis) andconditioned wire specimens (700, 1100, 1240, 1400 and 1500 MPa on thex-axis) The conditioning resulted in an upward cycle life shift of atleast 54% at the 0.95% alternating strain test levels respectively at aconditioning load level of 1240 MPa. An overall upward trend in lifetimefor a given test strain level was observed for increasing conditioningload through 1500 MPa. Most samples of the material conditioned atgreater than 1240 MPa survived more than 10⁶ cycles and were stillrunning at the time of conclusion of the experiment for test strainlevels below 0.80%.

FIG. 19( e) illustrates the observed tensile behavior during loadconditioning of each sample. In each case, an upper bound of the maximumvolume of retained martensite was calculated as described above based onthe ratio of isothermally non-recoverable strain to the strain length ofthe load plateau. FIG. 20 illustrates the positive correlation betweenisothermally non-recoverable strain and conditioning load.Non-recoverable strain was less than about 0.61% for all samplesconditioned below 1240 MPa resulting in a max. volume of retainedmartensite estimate of about 11% for the same samples loaded below 1240MPa.

In this Example, it has been demonstrated that mechanical conditioningof superelastic NiTi wire results in improved fatigue performance, whilemaintaining good mechanical properties. In addition, less than about 11%of the matrix was left in the martensite phase after load removal forconditioning below 1240 MPa with a concomitant maximum isothermallynon-recoverable strain of about 0.61%. Further, an increase in thestrain fatigue life of greater than 54% at 10⁶ cycles is observed inwire conditioned at 1240 MPa versus non-conditioned wire whilemaintaining good elastic properties suitable for said medical deviceapplications.

TABLE 3 Tensile Data for Various Wire Samples, see Examples 3-6. FIG.19(a) Strain level 1.1 Engr. 1.3 Engr. 1.4 Engr. 1.6 Engr. 1.7 Engr.Engr. Stress Engr. Stress Engr. Stress Engr. Stress Engr. Stress Strain(MPa) Strain (MPa) Strain (MPa) Strain (MPa) Strain (MPa) 0 0 0 0 0 0 00 0 0 0.0019 106.7 0.0020 108.5 0.0009 51.2 0.0016 86.8 0.0010 55.80.0154 655.3 0.0132 601.7 0.0047 295.3 0.0053 315.4 0.0048 287.4 0.0157614.5 0.0163 680.2 0.0134 626.4 0.0085 431.0 0.0096 461.7 0.0201 644.50.0180 591.1 0.0170 686.4 0.0139 630.5 0.0135 614.8 0.0523 643.1 0.0195635.8 0.0209 612.7 0.0189 642.5 0.0160 663.5 0.0796 639.8 0.0513 636.20.0305 636.1 0.0462 657.7 0.0393 621.6 0.0817 699.7 0.0747 638.4 0.0701634.9 0.0612 631.2 0.0579 624.2 0.0821 716.8 0.0784 635.4 0.0753 643.30.0766 642.1 0.0787 631.7 0.0826 700.5 0.0797 669.7 0.0793 690.4 0.0793665.2 0.0804 680.6 0.0746 431.8 0.0822 739.3 0.0826 755.0 0.0824 748.10.0856 809.0 0.0718 360.0 0.0899 933.9 0.0866 854.4 0.0872 870.6 0.09481036.8 0.0688 293.9 0.0968 1100.3 0.1006 1189.9 0.0953 1082.6 0.10831341.4 0.0646 222.5 0.0974 1116.1 0.1012 1201.9 0.1075 1351.4 0.12091500.8 0.0633 206.7 0.0976 1114.3 0.1019 1218.4 0.1082 1353.9 0.12231508.2 0.0618 190.8 0.0969 1092.6 0.1026 1234.8 0.1105 1400.7 0.12021427.7 0.0593 171.7 0.0921 891.0 0.1030 1245.6 0.1113 1406.2 0.11841353.7 0.0558 160.0 0.0801 460.0 0.1033 1243.3 0.1084 1286.7 0.10961012.9 0.0526 168.1 0.0774 381.8 0.1012 1157.5 0.1069 1225.5 0.0940463.0 0.0461 160.4 0.0742 303.2 0.0985 1043.9 0.1007 970.9 0.0798 100.30.0393 178.4 0.0705 225.0 0.0887 662.5 0.0788 240.8 0.0734 4.9 0.0288170.2 0.0667 165.6 0.0791 358.0 0.0725 116.4 0.0729 0.1 0.0233 169.80.0615 113.5 0.0745 246.1 0.0640 33.1 0.0727 0.0 0.0113 167.4 0.0584106.0 0.0691 149.6 0.0590 39.1 0.0063 169.4 0.0540 111.2 0.0651 103.40.0511 33.0 0.0019 0.0 0.0426 113.7 0.0585 84.8 0.0440 35.5 0.0309 112.00.0520 91.9 0.0352 38.7 0.0074 113.2 0.0435 93.6 0.0265 35.2 0.0044 92.90.0335 95.8 0.0152 67.2 0.0030 53.3 0.0262 93.2 0.0115 56.6 0.0020 0.00.0174 81.9 0.0048 25.7 0.0122 99.3 0.0017 0.0 0.0089 111.8 0.0062 76.20.0029 17.0 0.0024 0.0 load plateau 0.0663 0.0690 0.0658 0.0654 0.0652length (strain) unload plateau 0.0627 0.0648 0.0627 0.0623 0.0071 length(strain) permanent set 0.0019 0.0020 0.0024 0.0017 0.0727 (strain) maxvolume 2.9% 2.8% 3.7% 2.6% 100.0% fraction martensite (%)

TABLE 4 Tensile Data for Various Wire Samples, see Examples 3-6. FIG.19(b) Strain level 2.1 Engr. 2.3 Engr. 2.4 Engr. 2.6 Engr. 2.7 Engr.Engr. Stress Engr. Stress Engr. Stress Engr. Stress Engr. Stress Strain(MPa) Strain (MPa) Strain (MPa) Strain (MPa) Strain (MPa) 0 0 0 0 0 0 00 0 0 0.0052 313.2 0.0009 49.5 0.0009 51.0 0.0007 37.9 0.0008 44.90.0107 540.7 0.0047 278.5 0.0047 277.4 0.0044 268.7 0.0046 274.8 0.0148647.1 0.0092 470.9 0.0093 470.0 0.0090 464.1 0.0092 469.4 0.0152 645.20.0165 668.0 0.0166 668.5 0.0163 671.1 0.0164 670.8 0.0159 650.8 0.0205641.1 0.0205 634.1 0.0203 632.2 0.0204 638.9 0.0164 654.0 0.0253 640.10.0253 627.5 0.0251 647.9 0.0252 647.7 0.0170 657.3 0.0584 649.5 0.0584644.5 0.0582 624.2 0.0583 641.6 0.0245 650.4 0.0694 646.7 0.0695 641.90.0692 642.2 0.0694 644.0 0.0684 641.0 0.0728 655.8 0.0728 658.1 0.0726666.4 0.0727 656.6 0.0734 637.0 0.0757 702.2 0.0757 701.7 0.0755 701.90.0756 701.7 0.0743 652.5 0.0782 749.1 0.0782 743.7 0.0780 747.0 0.0781746.1 0.0749 666.0 0.0901 1006.6 0.0901 1000.8 0.0899 1007.8 0.0900999.6 0.0763 697.5 0.0928 1059.7 0.0941 1088.6 0.0938 1097.3 0.09391084.7 0.0767 706.1 0.0932 1061.0 0.0970 1149.8 0.0967 1159.7 0.09691145.1 0.0767 696.1 0.0936 1070.4 0.0992 1189.1 0.1003 1230.4 0.10041213.8 0.0653 386.6 0.0944 1087.2 0.1000 1195.9 0.1059 1326.9 0.10601310.6 0.0603 304.9 0.0951 1103.2 0.1007 1210.8 0.1084 1345.7 0.11141384.3 0.0553 260.5 0.0955 1109.7 0.1014 1226.5 0.1091 1352.0 0.11691440.2 0.0506 244.2 0.0955 1104.3 0.1021 1239.8 0.1099 1363.8 0.11991442.0 0.0446 249.3 0.0953 1098.4 0.1022 1240.6 0.1102 1369.1 0.12021438.8 0.0361 260.2 0.0919 961.0 0.1022 1236.0 0.1106 1377.1 0.12191454.3 0.0183 258.0 0.0877 800.9 0.1022 1231.9 0.1112 1385.9 0.12391475.3 0.0158 267.6 0.0867 762.9 0.1016 1210.2 0.1113 1388.1 0.12601488.8 0.0086 232.7 0.0827 626.9 0.0976 1045.7 0.1118 1396.0 0.12791497.3 0.0071 218.2 0.0792 518.0 0.0941 905.5 0.1123 1401.5 0.13091499.0 0.0053 192.6 0.0759 428.1 0.0908 780.5 0.1128 1399.3 0.12871417.2 0.0028 113.0 0.0707 313.8 0.0856 601.6 0.1129 1397.9 0.12521281.9 0.0008 22.5 0.0671 255.5 0.0820 493.4 0.1106 1307.1 0.1162 952.80.0007 0.0 0.0634 210.8 0.0783 393.2 0.1087 1230.3 0.1114 787.8 0.0588185.1 0.0737 291.6 0.1041 1047.3 0.1060 614.2 0.0540 179.0 0.0689 212.50.0993 864.9 0.0943 298.8 0.0517 180.0 0.0666 184.7 0.0970 781.4 0.0908222.1 0.0469 186.4 0.0618 148.3 0.0922 617.6 0.0864 141.9 0.0323 190.50.0472 152.9 0.0851 407.8 0.0820 78.7 0.0205 191.7 0.0354 166.8 0.0777237.4 0.0779 35.3 0.0096 180.8 0.0245 165.2 0.0735 169.3 0.0750 14.60.0069 148.3 0.0218 154.2 0.0679 108.1 0.0708 3.0 0.0038 55.4 0.0187152.5 0.0618 81.8 0.0660 10.7 0.0017 0.2 0.0166 155.5 0.0568 91.8 0.062714.5 0.0009 0.0 0.0158 163.8 0.0541 94.1 0.0506 16.1 0.0110 152.6 0.0508101.1 0.0335 14.0 0.0064 104.3 0.0324 101.4 0.0260 5.4 0.0049 55.60.0154 98.9 0.0231 1.2 0.0039 13.1 0.0076 49.0 0.0223 0.0 0.0035 0.00.0039 0.2 0.0018 0.0 load plateau 0.0601 0.0592 0.0592 0.0592 0.0592length (strain) unload plateau 0.0596 0.0662 0.0654 0.0717 0.0485 length(strain) permanent set 0.0007 0.0009 0.0035 0.0018 0.0223 (strain) maxvol. 1.2% 1.5% 5.9% 3.1% 37.6% fraction martensite (%)

TABLE 5 Tensile Data for Various Wire Samples, see Examples 3-6, FIG.19(c) Strain level 3.1 Engr. 3.3 Engr. 3.4 Engr. 3.6 Engr. Engr. StressEngr. Stress Engr. Stress Engr. Stress Strain (MPa) Strain (MPa) Strain(MPa) Strain (MPa) 0 0 0 0 0 0 0 0 0.0003 16.0 0.0009 52.2 0.0007 37.70.0006 32.1 0.0048 237.0 0.0047 230.8 0.0044 239.5 0.0043 232.3 0.0105450.1 0.0095 406.1 0.0092 420.0 0.0091 410.1 0.0138 550.1 0.0122 509.70.0119 525.8 0.0118 512.5 0.0173 544.5 0.0151 556.9 0.0149 570.7 0.0147563.6 0.0358 527.9 0.0305 529.6 0.0303 539.6 0.0302 550.0 0.0618 529.20.0522 523.1 0.0519 538.8 0.0518 513.0 0.0733 549.4 0.0618 523.1 0.0615543.5 0.0614 530.4 0.0748 571.5 0.0630 510.2 0.0628 534.2 0.0627 539.50.0785 631.8 0.0662 525.3 0.0659 548.4 0.0658 522.6 0.0802 661.4 0.0687529.5 0.0684 552.7 0.0683 507.3 0.0807 664.3 0.0716 543.2 0.0713 567.40.0712 536.6 0.0813 673.7 0.0755 590.1 0.0753 609.2 0.0752 591.3 0.0820686.2 0.0797 668.3 0.0794 670.0 0.0793 663.0 0.0827 700.6 0.0841 767.60.0838 761.7 0.0837 758.2 0.0832 709.6 0.0901 908.9 0.0899 899.3 0.0897896.7 0.0831 701.3 0.0969 1060.2 0.0976 1069.0 0.0975 1068.7 0.0801591.9 0.0973 1059.3 0.1001 1118.4 0.1000 1118.5 0.0746 411.1 0.09791070.4 0.1043 1193.2 0.1045 1200.1 0.0711 316.3 0.0983 1078.4 0.10461184.5 0.1075 1244.7 0.0666 219.2 0.0988 1089.9 0.1051 1191.0 0.11121293.0 0.0601 128.9 0.0994 1103.9 0.1059 1204.6 0.1166 1346.2 0.0536100.1 0.0998 1107.3 0.1066 1218.6 0.1186 1337.7 0.0444 100.8 0.09921078.3 0.1076 1236.3 0.1197 1345.4 0.0424 102.4 0.0975 1004.8 0.10771238.6 0.1199 1348.3 0.0371 108.2 0.0931 821.3 0.1079 1235.3 0.12051355.8 0.0306 107.0 0.0877 611.1 0.1080 1232.6 0.1212 1364.3 0.0249107.5 0.0829 445.7 0.1034 1040.9 0.1218 1370.6 0.0209 111.3 0.0796 343.70.1001 903.4 0.1223 1374.5 0.0159 109.8 0.0754 234.2 0.0959 738.9 0.12281378.9 0.0101 100.7 0.0706 134.6 0.0911 563.4 0.1235 1383.3 0.0059 78.90.0671 80.0 0.0876 445.1 0.1239 1386.1 0.0031 43.4 0.0648 53.3 0.0853374.5 0.1242 1387.8 0.0021 17.8 0.0640 45.1 0.0845 350.1 0.1244 1388.40.0006 2.2 0.0627 34.6 0.0832 314.7 0.1245 1389.3 0.0003 0.0 0.0596 17.80.0801 233.3 0.1249 1391.5 0.0567 12.8 0.0772 167.0 0.1253 1393.4 0.053115.1 0.0736 100.4 0.1258 1395.6 0.0510 15.6 0.0716 68.2 0.1261 1396.70.0485 18.0 0.0691 36.9 0.1264 1398.2 0.0458 22.0 0.0664 11.4 0.12671399.6 0.0446 23.7 0.0651 2.6 0.1269 1400.2 0.0433 24.1 0.0639 2.00.1271 1400.6 0.0333 26.7 0.0539 2.0 0.1281 1399.7 0.0306 26.5 0.05112.0 0.1256 1303.2 0.0279 27.4 0.0484 2.0 0.1229 1197.3 0.0217 23.00.0422 2.0 0.1166 960.1 0.0167 22.8 0.0372 13.7 0.1116 779.5 0.0140 18.30.0345 17.6 0.1089 686.3 0.0102 9.8 0.0307 19.4 0.1052 563.2 0.0063 0.40.0268 19.6 0.1012 441.7 0.0019 0.5 0.0224 18.1 0.0968 318.9 0.0013 0.00.0180 14.7 0.0924 208.9 0.0141 8.3 0.0885 121.9 0.0116 3.2 0.0860 73.70.0109 0.0 0.0837 34.4 0.0806 1.2 0.0731 0.0 load plateau 0.0664 0.06460.0633 0.0675 length (strain) unload plateau 0.0598 0.0658 0.0606 0.0075length (strain) permanent set 0.0003 0.0013 0.0109 0.0731 (strain) maxvolume 0.4% 1.9% 17.3% 100.0% fraction martensite (%)

TABLE 6 Tensile Data for Various Wire Samples, see Examples 3-6, FIG.19(d) Strain level 4.1 Engr. 4.3 Engr. 4.4 Engr. 4.6 Engr. 4.7 Engr.Engr. Stress Engr. Stress Engr. Stress Engr. Stress Engr. Stress Strain(MPa) Strain (MPa) Strain (MPa) Strain (MPa) Strain (MPa) 0 0 0 0 0 0 00 0 0 0.0063 344.2 0.0037 195.8 0.0029 158.3 0.0024 127.7 0.0022 119.30.0108 520.9 0.0059 276.6 0.0050 239.7 0.0046 229.8 0.0043 211.8 0.0133606.8 0.0076 335.0 0.0066 296.5 0.0063 294.0 0.0059 270.9 0.0158 644.60.0098 410.4 0.0087 372.3 0.0085 367.3 0.0080 341.4 0.0188 613.0 0.0119498.4 0.0108 461.2 0.0106 448.9 0.0101 426.9 0.0218 635.5 0.0139 574.40.0127 534.1 0.0126 529.7 0.0120 503.2 0.0258 633.3 0.0161 625.0 0.0148576.3 0.0148 596.8 0.0140 556.0 0.0308 623.0 0.0171 627.8 0.0158 585.00.0158 619.5 0.0151 569.8 0.0358 637.5 0.0206 631.5 0.0191 582.2 0.0193630.6 0.0184 576.1 0.0403 638.8 0.0249 596.2 0.0233 556.1 0.0236 596.00.0226 539.8 0.0485 606.7 0.0293 591.9 0.0275 561.4 0.0280 634.6 0.0268544.2 0.0668 614.5 0.0332 631.1 0.0312 581.7 0.0319 621.6 0.0305 578.10.0733 626.7 0.0403 622.8 0.0381 586.4 0.0390 628.7 0.0374 572.2 0.0745662.3 0.0561 614.6 0.0533 559.5 0.0548 622.2 0.0526 564.7 0.0750 671.10.0618 621.3 0.0587 560.3 0.0605 615.9 0.0580 571.1 0.0756 687.8 0.0633629.5 0.0602 580.9 0.0620 627.9 0.0595 572.1 0.0762 700.6 0.0661 599.20.0629 571.0 0.0648 608.4 0.0622 575.9 0.0767 711.5 0.0704 633.3 0.0670575.6 0.0691 617.3 0.0663 573.1 0.0771 720.3 0.0741 633.6 0.0706 570.70.0728 623.8 0.0699 579.1 0.0776 729.8 0.0778 696.1 0.0741 619.8 0.0765658.5 0.0734 595.5 0.0767 702.1 0.0808 771.2 0.0770 693.4 0.0795 735.80.0763 669.4 0.0740 605.1 0.0847 872.9 0.0808 788.7 0.0834 834.0 0.0801760.8 0.0700 483.3 0.0891 988.3 0.0850 892.8 0.0878 949.9 0.0843 863.60.0662 393.5 0.0915 1054.0 0.0873 948.0 0.0902 1012.6 0.0865 927.30.0615 306.7 0.0921 1062.2 0.0906 1042.0 0.0936 1104.8 0.0899 1014.40.0572 254.7 0.0926 1072.8 0.0937 1120.0 0.0969 1190.8 0.0930 1093.30.0537 231.9 0.0931 1088.1 0.0967 1190.4 0.1010 1290.1 0.0970 1193.30.0502 231.4 0.0936 1100.9 0.0972 1193.8 0.1042 1337.2 0.1005 1275.60.0452 231.0 0.0940 1111.5 0.0976 1200.4 0.1046 1350.1 0.1034 1335.90.0405 235.6 0.0943 1121.8 0.0980 1210.2 0.1050 1355.4 0.1063 1387.30.0392 232.5 0.0948 1132.9 0.0985 1223.1 0.1055 1366.6 0.1105 1447.90.0372 229.3 0.0945 1124.8 0.0990 1238.3 0.1061 1380.9 0.1120 1440.10.0347 228.3 0.0934 1071.7 0.0991 1239.9 0.1062 1382.7 0.1122 1439.30.0317 227.7 0.0917 997.2 0.0993 1247.0 0.1065 1389.6 0.1124 1442.20.0297 228.9 0.0895 906.9 0.0996 1251.1 0.1068 1394.0 0.1127 1443.30.0255 230.4 0.0869 802.3 0.0998 1257.5 0.1071 1400.9 0.1130 1448.40.0235 231.8 0.0852 736.5 0.1000 1260.7 0.1073 1406.4 0.1132 1451.10.0205 223.7 0.0815 606.5 0.0995 1240.7 0.1077 1414.0 0.1137 1457.80.0182 225.8 0.0798 549.2 0.0979 1160.1 0.1079 1419.0 0.1139 1459.90.0157 227.6 0.0772 472.3 0.0954 1044.6 0.1082 1421.9 0.1143 1467.00.0130 225.4 0.0752 417.1 0.0935 961.4 0.1083 1423.1 0.1145 1469.60.0092 222.3 0.0731 363.5 0.0914 874.3 0.1069 1357.3 0.1148 1474.90.0072 222.4 0.0707 310.6 0.0891 779.1 0.1045 1251.5 0.1151 1478.90.0020 150.0 0.0674 250.9 0.0860 659.2 0.1012 1110.7 0.1155 1484.50.0010 75.0 0.0657 225.5 0.0843 597.0 0.0995 1036.4 0.1157 1486.3 0.00090.0 0.0629 193.1 0.0816 502.9 0.0967 919.4 0.1161 1490.9 0.0607 172.90.0795 436.5 0.0945 834.3 0.1164 1492.7 0.0579 160.3 0.0768 357.3 0.0917725.4 0.1167 1496.4 0.0542 154.2 0.0733 264.8 0.0880 593.6 0.1172 1501.10.0494 161.9 0.0687 174.8 0.0832 439.3 0.1178 1506.8 0.0458 159.4 0.0652121.4 0.0796 335.0 0.1182 1510.6 0.0427 155.4 0.0622 91.0 0.0765 261.10.1186 1511.7 0.0395 158.5 0.0591 73.2 0.0733 197.1 0.1187 1478.2 0.0375157.3 0.0572 71.6 0.0713 163.8 0.1168 1425.1 0.0341 161.1 0.0539 69.40.0679 118.9 0.1134 1282.1 0.0302 166.9 0.0502 67.5 0.0640 85.0 0.10971123.1 0.0269 165.2 0.0470 71.3 0.0607 70.6 0.1066 994.2 0.0215 164.20.0418 77.9 0.0553 77.5 0.1014 788.7 0.0172 165.1 0.0377 75.9 0.051080.4 0.0972 634.1 0.0130 164.5 0.0337 75.8 0.0468 79.0 0.0932 495.00.0083 160.4 0.0291 80.2 0.0421 84.5 0.0887 350.3 0.0042 139.0 0.025277.5 0.0380 85.2 0.0847 241.7 0.0014 0.0 0.0229 76.3 0.0356 84.8 0.0824183.3 0.0208 74.3 0.0334 83.8 0.0803 136.0 0.0181 76.3 0.0306 79.30.0776 83.7 0.0145 72.2 0.0269 80.6 0.0741 25.2 0.0133 69.9 0.0256 75.60.0728 8.4 0.0124 66.8 0.0247 72.9 0.0720 0.0 0.0058 71.1 0.0178 79.50.0029 0.0 0.0148 74.8 0.0059 69.5 0.0035 29.8 0.0026 0.0 load plateau0.0617 0.0639 0.0615 0.0639 0.0623 length (strain) unload plateau 0.05630.0643 0.0623 0.0652 0.0021 length (strain) permanent set 0.0009 0.00140.0029 0.0026 0.0720 (strain) max volume 1.5% 2.1% 4.7% 4.1% 100.0%fraction martensite (%)

TABLE 7 Tensile Data for Various Wire Samples, see Examples 3-6, FIG.19(e) Strain level 5.1 Engr. 5.3 Engr. 5.4 Engr. 5.6 Engr. 5.7 Engr.Engr. Stress Engr. Stress Engr. Stress Engr. Stress Engr. Stress Strain(MPa) Strain (MPa) Strain (MPa) Strain (MPa) Strain (MPa) 0 0 0 0 0 0 00 0 0 0.0001 2.5 0.0002 11.8 0.0001 5.8 0.0002 11.1 0.0002 13.5 0.0036197.0 0.0033 182.0 0.0032 174.2 0.0033 191.0 0.0034 193.3 0.0068 321.20.0060 293.0 0.0059 283.2 0.0060 305.5 0.0061 307.4 0.0092 420.8 0.0081372.7 0.0080 362.4 0.0081 384.4 0.0082 387.0 0.0113 518.3 0.0100 450.10.0099 439.8 0.0100 460.9 0.0100 462.1 0.0137 602.3 0.0121 539.4 0.0120529.9 0.0121 549.4 0.0121 550.0 0.0149 606.7 0.0131 577.0 0.0130 567.90.0131 588.0 0.0132 588.3 0.0187 609.1 0.0165 633.9 0.0164 628.5 0.0165639.8 0.0165 634.2 0.0235 624.1 0.0206 623.3 0.0205 637.6 0.0206 646.80.0207 639.2 0.0283 620.0 0.0248 628.6 0.0247 620.0 0.0248 625.2 0.0248631.3 0.0329 628.2 0.0288 620.0 0.0286 624.8 0.0287 606.2 0.0288 619.60.0458 614.3 0.0400 606.6 0.0399 622.1 0.0400 615.0 0.0400 615.0 0.0537612.7 0.0469 615.0 0.0468 615.0 0.0469 620.9 0.0469 624.5 0.0647 636.00.0565 620.0 0.0564 620.8 0.0565 615.0 0.0565 610.8 0.0674 656.6 0.0588626.8 0.0586 603.6 0.0587 626.8 0.0588 611.5 0.0684 646.8 0.0623 610.00.0622 620.0 0.0623 634.8 0.0623 636.4 0.0694 637.6 0.0688 646.3 0.0686640.2 0.0687 662.6 0.0688 655.3 0.0703 640.1 0.0748 705.0 0.0747 701.00.0748 719.4 0.0748 715.1 0.0707 642.7 0.0777 749.6 0.0776 740.1 0.0777755.5 0.0777 752.3 0.0712 646.6 0.0808 805.1 0.0807 795.0 0.0808 805.00.0809 805.5 0.0717 651.1 0.0840 868.1 0.0839 858.0 0.0840 866.6 0.0840867.0 0.0723 658.7 0.0879 952.9 0.0878 941.8 0.0879 947.4 0.0880 947.70.0730 671.6 0.0925 1052.9 0.0924 1040.2 0.0925 1041.5 0.0925 1043.20.0737 682.3 0.0934 1060.4 0.0966 1128.3 0.0967 1125.7 0.0967 1127.90.0742 691.1 0.0939 1067.1 0.0999 1195.1 0.1004 1196.8 0.1005 1199.40.0748 701.8 0.0944 1077.3 0.1004 1187.8 0.1044 1265.3 0.1044 1268.10.0751 703.7 0.0949 1087.6 0.1009 1194.8 0.1081 1324.6 0.1082 1325.40.0753 704.0 0.0956 1100.8 0.1015 1205.8 0.1109 1346.4 0.1130 1386.10.0754 703.6 0.0958 1103.6 0.1021 1217.1 0.1115 1347.3 0.1175 1432.40.0735 637.3 0.0959 1101.9 0.1024 1223.2 0.1118 1350.7 0.1196 1448.70.0701 525.6 0.0960 1104.0 0.1028 1230.1 0.1122 1356.5 0.1215 1450.50.0679 461.5 0.0960 1102.9 0.1030 1234.8 0.1124 1358.8 0.1217 1440.60.0653 391.9 0.0961 1103.0 0.1031 1235.8 0.1127 1363.5 0.1221 1438.20.0627 331.9 0.0940 1019.0 0.1032 1233.0 0.1130 1367.1 0.1224 1439.40.0603 285.9 0.0919 933.0 0.1033 1234.5 0.1133 1372.6 0.1226 1441.90.0567 231.8 0.0888 809.1 0.1034 1234.1 0.1137 1379.2 0.1231 1446.70.0538 200.7 0.0863 714.9 0.1034 1234.0 0.1141 1383.4 0.1234 1450.90.0507 182.2 0.0836 618.5 0.1009 1131.5 0.1144 1388.9 0.1237 1455.60.0473 177.7 0.0807 522.3 0.0980 1009.8 0.1148 1393.5 0.1241 1458.70.0445 174.5 0.0782 446.6 0.0955 908.5 0.1151 1399.1 0.1245 1463.50.0409 177.4 0.0751 361.8 0.0924 786.7 0.1154 1401.2 0.1249 1467.60.0387 171.2 0.0732 316.3 0.0905 716.6 0.1155 1399.1 0.1251 1470.90.0351 164.4 0.0701 250.1 0.0874 605.7 0.1157 1400.1 0.1256 1474.30.0315 169.3 0.0669 195.9 0.0843 502.9 0.1158 1399.5 0.1260 1479.30.0277 164.7 0.0636 151.1 0.0809 403.7 0.1146 1351.2 0.1264 1483.60.0248 165.0 0.0611 125.9 0.0784 336.9 0.1121 1247.1 0.1267 1485.30.0215 162.7 0.0582 105.2 0.0755 267.7 0.1092 1127.3 0.1271 1489.00.0181 153.3 0.0553 100.4 0.0726 208.4 0.1063 1009.9 0.1275 1491.00.0152 139.7 0.0528 95.4 0.0701 165.6 0.1038 911.7 0.1279 1493.8 0.0119123.9 0.0499 106.2 0.0672 125.2 0.1009 800.8 0.1282 1494.9 0.0083 98.70.0467 102.2 0.0641 92.7 0.0978 687.0 0.1287 1498.3 0.0042 28.0 0.043293.8 0.0605 70.8 0.0942 565.7 0.1291 1501.2 0.0021 0.0 0.0413 104.60.0586 68.3 0.0923 505.0 0.1294 1502.6 0.0392 96.9 0.0566 64.6 0.0903441.0 0.1296 1503.6 0.0374 96.6 0.0547 67.4 0.0884 386.6 0.1298 1503.40.0344 94.9 0.0518 62.4 0.0855 308.1 0.1301 1503.0 0.0315 105.2 0.048969.0 0.0826 237.9 0.1304 1503.2 0.0286 103.0 0.0459 72.7 0.0796 176.40.1307 1503.1 0.0257 91.7 0.0430 67.4 0.0767 124.0 0.1289 1432.0 0.022889.8 0.0401 59.2 0.0738 81.0 0.1259 1317.7 0.0192 96.3 0.0366 59.50.0703 41.4 0.1224 1180.7 0.0165 85.6 0.0339 63.0 0.0676 20.2 0.11971077.6 0.0130 68.3 0.0303 67.8 0.0640 3.5 0.1161 946.0 0.0094 48.80.0268 60.4 0.0605 5.2 0.1126 818.7 0.0074 28.9 0.0247 59.9 0.0584 2.30.1105 746.4 0.0059 9.8 0.0232 64.1 0.0569 0.6 0.1091 696.9 0.0040 0.00.0214 62.4 0.0551 5.8 0.1072 634.9 0.0189 59.6 0.0526 3.6 0.1047 555.10.0153 46.2 0.0490 12.5 0.1024 485.2 0.0116 27.2 0.0453 14.5 0.0982366.4 0.0061 0.0 0.0398 9.6 0.0934 244.1 0.0342 14.2 0.0897 160.5 0.030513.6 0.0861 92.3 0.0244 11.7 0.0828 38.4 0.0217 0.0 0.0805 7.3 0.07840.0 load plateau 0.0617 0.0617 0.0556 0.0556 0.0617 length (strain)unload plateau 0.0546 0.0596 0.0610 0.0485 0.0021 length (strain)permanent set 0.0021 0.0040 0.0061 0.0217 0.0784 (strain) max volume3.3% 6.5% 11.1% 39.0% 100.0% fraction martensite (%)

IV. APPLICATIONS

Wires made in accordance with the present disclosure are susceptible ofa variety of applications including, but not limited to the applicationsdetailed below. Exemplary applications of wires in accordance with thepresent disclosure are set forth below, and shown generally in FIGS. 18(a)-19(b).

In some cases, a wire may have no remaining biased curvature, such as ina percutaneous transluminal coronary angioplasty (PTCA), steerable, andtorque whip free guidewire application, or for a torque transmissionwire for coronary plaque removal, for example.

Wire products used for medical devices as discussed herein willtypically be subjected to mechanical conditioning in accordance with thepresent disclosure prior to integration into a medical device. However,it is contemplated that wire products may alternatively be installedinto, or at least partially configured as, a medical device prior tosubjecting the wire product to mechanical conditioning, followed byconducting the mechanical conditioning on the wire product after same isinstalled into, or at least partially configured as, a medical device,in order to impart benefits as disclosed herein.

A. DFT® and Other Composite Wire Materials

Wires disclosed herein may be used for composite wire products, such asshown in FIGS. 21( a)-(b). Composite wire 300 includes an outer shell302 made of a first material, and a core 304 comprising at least onecore segment of a second material, and optionally, additional coresegments of third or more materials. Outer shell 302 may be made of awire in accordance with the present disclosure, and core 304 may have avariety of desired properties, such as resistance, radiopacity, or anyother property.

Thus, composite wire 300 may confer the benefits of load-conditioned andtherefore fatigue damage resistant outer shell, such as fatiguestrength, low or zero permanent set, etc, as described above by applyinga suitable conditioning load to the wire product prior to installationwithin a medical device and/or configuration as a medical device, whilealso having other properties associated with the second materialcomprising core 304. An exemplary composite wire product is DFT®,available from Fort Wayne Metals Research Products Corp. of Fort Wayne,Ind.

B. Shape Memory Devices

1. Wire-Based Stents

Referring to FIG. 22( a), a tissue scaffold or vessel stent device 370is shown which is made from one or more wires 372 made in accordancewith the present process, which are braided, knitted, or otherwiseformed together to produce the generally cylindrical cross-sectionalshape of device 370.

Referring to and FIG. 22( b), a tissue scaffold or vessel stent device370′ is shown which is made from one or more wires 372′ made inaccordance with the present process, which are knitted together to formthe generally cylindrical cross-sectional shape of device 370′.

Upon release from the delivery catheter, stents move to some degree,dependent on the relative vessel and device compliance, with the arterydue to fluctuations in blood pressure, arterial vessel smooth musclecontraction and dilation, and due to general anatomical movement. Suchmechanical displacement results in cyclic straining of wires 372, 372′comprising the structure of stent 370, 370′ structure.

Non bioerodable tissue scaffolds or stents are generally implantedpermanently, and therefore should be able to withstand millions ofmechanical load cycles without losing structural integrity due tomechanical fatigue.

Stents 370, 370′, which are constructed from wires 372, 372′ made inaccordance with the present process, possess a high degree of resistanceto fatigue damage and thus offer optimized performance as compared toconventional stents made with wires having lower fatigue strength.

2. Blood Filters

Referring still to FIGS. 22( a)-(b), devices 370, 370′ may also take theform of a blood filter which is made from one or more wires 372, 372′made in accordance with the present process, which are braided, knitted,or otherwise formed together to produce the generally cylindricalcross-sectional shape of devices 370, 370′. In this respect, many bloodfilter devices may be similar to braided, knitted, or laser-cut stents,and many interior vena cava (IVC) filters may be shaped asumbrella-shaped devices. In use, the superelastic characteristic of thedevice is utilized in that the device is inserted into a blood vesselvia catheter in a collapsed condition and is deployed by expansion intothe blood vessel, where the device captures and/or redirects largerblood clots from critical anatomical organs or regions. In use,particularly in permanent or non-retrievable devices, the device may besubjected to repeated, movements such that high fatigue strength isdesired.

While this invention has been described as having a preferred design,the present invention can be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

1. A medical device comprising a wire made of a nickel-titanium shapememory material, said wire having a fatigue endurance exceeding 0.95%strain amplitude at greater than 10⁶ cycles.
 2. The medical device ofclaim 1, wherein said wire has a fatigue endurance exceeding 1.1% strainamplitude at greater than 10⁶ cycles.
 3. The medical device of claim 1,wherein said wire has a fatigue endurance exceeding 1.1% strainamplitude at greater than 10⁹ cycles.
 4. The medical device of claim 1,wherein said wire has a residual strain of less than 0.25% after beingsubjected to engineering strain of at least 9.5%.
 5. A medical devicecomprising a wire product made of a shape memory material, said shapememory material having a plurality of defects, said wire productsubstantially comprised of said shape memory material in a primary phaseand including portions of said shape memory material comprising asecondary phase at localized regions disposed proximate respective saiddefects, with at least some of said secondary phase portions separatedby said primary phase.
 6. The medical device of claim 5, wherein saidshape memory material is a nickel-titanium shape memory material, saidprimary phase is an austenite phase, and said secondary phase portionscomprise a martensite phase.
 7. The medical device of claim 5, whereinsaid secondary phase portions together comprise less than 15% of theshape memory material, by volume.
 8. The medical device of claim 5,wherein said shape memory material is a nickel-titanium shape memorymaterial, and said wire product has a fatigue endurance exceeding 0.95%strain amplitude at greater than 10⁶ cycles.
 9. The medical device ofclaim 5, wherein said shape memory material is a nickel-titanium shapememory material, and said wire product has a fatigue endurance exceeding1.1% strain amplitude at greater than 10⁶ cycles.
 10. The medical deviceof claim 5, wherein said shape memory material is a nickel-titaniumshape memory material, and said wire product has a fatigue enduranceexceeding 1.1% strain amplitude at greater than 10⁹ cycles.
 11. Themedical device of claim 5, wherein said wire has a residual strain ofless than 0.25% after being subjected to engineering strain of at least9.5%.
 12. The medical device of claim 5, wherein said wire product isselected from the group consisting of wire having a circularcross-section, wire having a non-circular cross-section, cable, coil,and tubing.
 13. A method, comprising the steps of: providing a wireproduct made of a shape-set, shape memory material; mechanicallyconditioning the wire product by: applying an engineering stress between700 MPa and 1600 MPa; and releasing the applied engineering stress; andincorporating the wire product into a medical device.
 14. The method ofclaim 13, wherein said mechanical conditioning step occurs prior to saidincorporation step.
 15. The method of claim 13, wherein said mechanicalconditioning step occurs after said incorporation step.
 16. The methodof claim 13, wherein said mechanical conditioning step comprises:applying an engineering stress between 900 MPa and 1450 MPa; andreleasing the applied engineering stress.
 17. The method of claim 13,wherein said mechanical conditioning step comprises: applying anengineering stress between 1100 MPa and 1350 MPa; and releasing theapplied engineering stress.
 18. The method of claim 13, furthercomprising repeating said mechanically conditioning step at least once.19. The method of claim 13, wherein the shape memory material is anickel-titanium shape memory material.
 20. The method of claim 19,wherein said mechanical conditioning step is conducted below amartensite deformation temperature (M_(d)) of the nickel-titanium shapememory material.
 21. The method of claim 19, wherein said mechanicalconditioning step further comprises: applying the first force to thewire product in an environment having a temperature T, whereinT=A _(f)±50° C., wherein A_(f) is the austenite transformation finishtemperature of the nickel-titanium shape memory material.
 22. The methodof claim 13, wherein the wire product is selected from the groupconsisting of wire having a circular cross-section, wire having anon-circular cross-section, cable, coil, and tubing.